My skids are not allowed to hold the baby....they are 10 and 11. My child is not a doll to be played with. They each held him a few time while seated on the couch under direct supervision. That's it.

And I think a little curiosity about diaper changed is normal at first....but if he's still fixated 6 weeks later I think its a bit weird. Why so fascinated with poop or a babies penis? My 3 yrbold was interested at first but after the third or fourth time he went about his merry way.

Overall I think it's a good thing if SS shows an interest in your baby and if that interest is supported will probably reduce friction and resentment and make for happier family time. Your DH will be sensitive about it because he wants his sons to bond and will be aware that the arrival of a new baby will be a big adjustment for SS, he wants him to feel included not alienated. I would leave a sensible 9 year old to hold my baby, as long as they were seated, the head was supported by a pillow or cushion, I was not too far away, and they understood they should not try to get up and carry baby around. The interest in baby care and changing seems perfectly normal, and all signs that he wants to be included. That may get a little irksome at times but its a lot better than withdrawal, resentment or acting out. Maybe make him feel useful and get him to pass you stuff, the interest will probably wear off soon anyway.