I do not negotiate with terrorists. 

I think you are giving him excellent advice. Counselor, lawyer, and then file. I would also go get my boat back if I was SO. If he refuses then file a stolen property case with the police. 

When my BS was 15 years old, he decided he was living with his dad and wasn't going to talk to me again.  He did spend about a year being angry at me.  To this day, I have no idea why he was angry at me.  I can assume that his father made him a lot of promises that he never kept.  Of course, my BS had to find out the hard way. I would text or try to call from time to time.  I didn't beg him to come back to my house. Now, he in his 20's and we do have a relationship now.  

I would suggest that you DH give him a little space before he starts give anything back unless SS or BM purchased it.  Make sure SS doesn't have a key to the house.  He may come by and take more than just his things.  

SD14 is refusing to talk with DH, apparently because she didn't like being told 'no' and that he wasn't agreeing to how she wanted to conduct herself in our home or with DH.  So she left for BMs and didn't come back.  It's been since the end of April.  SD14 has not said one word.  SD12 is coming over as usual and seems happy.

SD14 went to one counseling session.  Nothing further came of that.  No lawyers are involved and I'm just hoping that BM doesn't file for addisional CS (I don't think she will but as we all know that could happen). 

DH, with the help of HIS counselor, drops a line to her now and then, just to say hello.  He doesn't put himself out there or beg (giving her exactly what she wants, all the power).  He's gonig to invite her on our summer trip but we don't think she'll answer him.  (whew) and her stuff is boxed/bagged up, her room has been re-painted and will be re-done this fall and given to SD12.

I don't bring up SD14 unless I have to.  Ex:  I had to tell DH he needed to buy more sheets for the bed because she ruined all of them that she had by either not washing them or spilling stuff on them.  I offer him my support and listen but offer no opinion unless asked.

DH lent SD an expensive instrument for band.  Eventually he knew he would never get it back so instead of getting her anything for graduation from HS, he gave her a card with a note that said she could keep the instrument.  I was good with it because, 1. it didn't affect our bank account, and 2, she probably would have damaged it if forced to return it.  If she wanted cash she could sell it.  DH is estranged from this SD, she attacked him at age 16 and has yet to apologized and make amends.  She is 22 now.  

Not sure about this boat, but maybe that would be a solution.  Or since he is in mind set of bargaining, counter back with a list of things that DH wants back.  Teach the little shit about negotiation.  

my DH and I are going through the same thing.  OSs21 stopped communicating with DH off and on for past 5 years and finally stopped for good a year ago.  Mainly because DH stopped chasing him and begging for him to see and talk to him And because DH told him no when OSS tried to guilt him into doing something OSS wanted.   Now MSS17 is following in his footsteps.   MSS shunned DH for made up reasons.  The real reason is because they get praise and attention from BM for doing it.  And without DH in the picture, they can run the show at BMs.    I told DH years ago to stop begging for their love and attention.  He finally realized that no matter what he did and even if he caved to they every demand the SSS were going to treat him terrible and only come by when they want something.  DH recently stopped thei phones and told them he was done trying which caused a slew of nasty calls and texts from MSS.   DH has finally put his foot down and is staying strong.  

Do NOT beg or plead the kid to come around as you are giving them power.  I agree with not negotiating with terrorist!!!!

You know.. I would be very tempted to tell BM  that you are not going to communicate with your son through her.  He is still a child and I am still the father... not talking with me is not an option that I am entertaining.  But if this is how he wants to play it... he better be prepared for adult sized consequence... and her too. (ie court etc.. for PAS and not returning the boat etc..)

Man. This all stinks so bad.

Our OSD played games like this starting at about age 14 through age 18 when she quit coming over and quit speaking to DH for almost a year. 

She's come back around and they do have a relationship now (she's 21)...but DH did NOT chase her. He definitely wanted to and talked about things he could get her / do for her a lot...but he was starting a business at the time and had zero money, so thankfully he was saved from actually making a fool of himself by trying to buy back her love. After about a year, she reached out to him on her own...

At this point, as Dad has not spoken to nor seen his son, you have no idea what SS is actually agreeing to, saying, or demanding.... only what BM is telling Dad is going on.

It's a 35 minute drive? Why hasn't Dad tried driving over and speaking to his son in person? BM isn't withholding her son from visitation (contempt) unless Dad has shown up at the door and tried to pick the kid up. 

How did the boat get to SS's home? Did the kid come , hook it up and drive away with it? Why doesn't Dad just drive over and get his boat? It's ridiculous to call and claim stolen property unless the kid literally snuck the boat away without Dad's permission. And if Dad hasn't asked the kid for it back, where is the stealing? 

Just my two cents, but its not the father the kid is avoiding here. It's you and your daughter. The camping friend invite and then basic take away long with your daughter running around asking one of SS's friends why that friend didn't want to go, was your daughter sticking her nose in where it didn't belong. Clear and unmistakable rules and guidelines should have been put out as to who the kids could invite (aka who Dad found and would find acceptable). 

I wouldn't pressure nor go through court to force this kid to start doing visits again. The visits wont go well. I'd encourage Dad to go personally face to face talk with his son and follow up with Dad spending sometime with his son outside your home. Example, Wednesday evening dinner together in SS's town. Dad occasionally swinging by to watch a sport SS participates in. Just something that keeps Dad and SS involved with each other and talking. 

This kid feels hurt and is angry. He will turn rebellious nd really act out if you force him to visit your home. Sure your Dh is upset and anxious over what is occurring , but this type of thing happens fairly often in divorced families where the kids do visitation and they suddenly turn 16-ish. They work. They have friends. They don't like being told 'you are still is kid and you will follow my orders, especially when the person giving the demands is only a EOWE parent. Your Dh put the kid in an embarrassing position by letting SS invite a friend, then wanting kid to revoke invitation, then let the friend come but only during the day. No, I'm not saying the questionable friend should have come, but surely Dad could have started out with given the SS several friends on a list which would be gladly acceptable to select him.