Probably he is a bit spoiled because he's been sick; and he was tiny and you needed to protect him at the time.
Lucy got the same treatment; her brother was a lot bigger than her (a Westie) and she had liver issues and I was always so worried about her that I definitely spoiled her.
However, my Westie is very dominant and he still maintains his role as Alpha.
One thing I would say is do not put him on his back; that is the 'alpha roll' and it has been shown to do more harm than good. It could be making him even more aggressive. I tried it with Ringo before I knew better and I really think it made him more aggressive toward other dogs.
Ringo was very aggressive when we first brought Lucy home; he never hurt her but he would try to pounce on her if she got the good toys, etc. First, we took up all the toys and bones off the floor so there was nothing to fight over. Slowly we re-introduced them. If Ringo got snarly with Lucy for say, sitting in my lap; he got put in another room for a while. He figured it out rather quickly.
I hope more experienced folks will jump in too
__________________  Mommy to Lucy, Ringo, and Matthew |