Don't forget that the 10 month old adds another symptom here....fatigue. I'm sure you're exausted chasing him around
If part of the boyfriend thing (not judging) is that he's still new and exciting, sex with your husband may simply be same-o same-o in comparison.
Add a little spice on the home front....get a sitter or send the baby to his grandparents, and the two of you go out for a date night. Whether it's something as simple as dinner and a movie, or if you two get all dressed up and paint the town red. The best idea I can think of is, have one of 'those dates'...think back to the beginning and do something that you two used to love early on. For me and T, it's going to the movie theater, especially on a weekday afternoon. We'd get the theater basically to ourselves because it was a work day and....nuff said
Another thought....and again, please, I am not trying to judge anything, I just wonder if it might be a possibility. You mention that your boyfriend is having issues with lasting due to inexperience, and that he himself hasn't been a part of the arrangement for long. How 'old' is the arrangement on the part of you and your husband? Is it fairly new, or have you approached it this way all along? Is it at all possible that you or your husband could be changing your mind about it, and it's manifesting this way?