He may need to grow up BUT he may also need some reassurances & quality time (or more of quality time) with you to help ease his bruised ego.
Think, is there something you, and ONLY YOU, do for him that you haven't done for a while? Is there any way you could do that thing for him some time in the near future? It will make him feel loved and special and may just help alleviate some of those feelings of uselessness, feeling unloved and unwanted AND it will give the two of you a special bonding time specifically for the two of you.
There are as many ways to do polyamory as there are people practicing it!