W.B.
Hey A.,
If you've gone out of your way to keep communication open with your son about his feelings, you're a huge step closer to a good resolution. But don't forget, you're the parent in the situation. Don't let him forget that, either. That doesn't necessarily mean you have get nasty with him, but firmly keeping the boundaries in line will help during times of change, particularly with new family members added into the mix.
Being firm, but lovingly keeping the existing boundaries in tact will help him through this time. Kids act up when something is bothering them. Many times they aren't able to put their finger on exactly what the heck is bothering them or aren't able to vocalize it, but something is just kind of "off".
It may just take him a little time to work through his feelings before he's ready to share them with you. Making sure to keep those communication lines open will help him feel like he can approach you when he's ready. Sometimes just recognizing when he's ready is the toughest part.
Oh, and ALL kids are ecstatic when they're being bought something or getting to do whatever they want. That's totally normal! =)
Good luck, A.!
~W.