Don't need to do anything, give him a big hug and tell them that you and BB will fully support him and stand by his side. You, him and bb are 一個個體,會齊上齊落。
I understand he must be feeling very upset (because the way his mum has let him down) and guilty towards you and bb (that you have to live in a rented flat etc), try and assure him that you are still very happy, and bb is too (show him bb is kicking you actively)
This is a very difficult time for him and for you - but the good side is this has made you two stronger!