When you say "what we get for our child," what exactly do you mean?  Are you talking about the child credit?  Child care credit?  Something else?  I'm assuming you have separate finances (or there'd be no need to split the tax refund); how do you split household costs?  Does your husband pay all expenses for SS?  Does your husband handle 62.5% of expenses and you the remainder, since he is responsible for 2.5 of the members of the household and you for 1.5?  

If your husband pays all of SS's expenses, including his share of household expenses, then I think he should get the tax refund attributable to SS.  If you split those expenses, then you should split the tax refund.  This assumes, of course, that you both had income and paid taxes, since there would need to be earnings/taxes to take advantage of the non-refundable portion of the child credit.

If your SO is paying all of SS's expenses and doesn't get any CS from BM, then I'd say it's fair for him to keep the whole return (unless they have a court order saying otherwise). If she were paying CS and maintaining her visitation schedule, then I'd say it's fair to split, though it would be better (in my opinion) to switch each year of who claims him.