L.,
I know you are more comfortable with where things are than when you first posted, but this concerned me:
>>If he is holding the baby and I walk by and start to
>>talking to him then my husband will turn the baby toward
>>him and start tickling him or whatever
You are with your baby more often, **and this may not be the case, but I think you need to do more with your son as far as interactive activities. If you don't already: Name stuff around the house, down to the last weird objects (door handle, lock, wood grain, color, hard, hollow noise, etc for a door description.) Getting him involved with interacting with YOU will help this part of the 'game' it looks/sounds like your husband is playing.
I can't express everything I want in this, but I have a husband who has been doing something very similar. DS is overjoyed when DH comes home (to the point where he stays up later at night to see him come home), and while there are other issues making what I have a bit different from what you seem to have, it's still an issue with ME. I don't want my child to grow up thinking all the fun comes from Dad. It's an equal expectation that fun comes from both parents, not work from Mom and fun from Dad. That's when respect and responsibility start to go out the window, and most people don't recognize it or want to try to get it back.
Go with your gut, you're not paranoid.
Play the little attention and focusing games with your son. Don't leave him crying anywhere to get him used to it. (not saying you do, but it doesn't help what you are trying to change). Avoid TV if you can (if you have to keep it on for your sibling, I understand) it isn't good for your baby.
Soooo much more I want to impart and my own little one is stirring.
I wish you the best, and hope for the best for your son. I hope I am totally off base!
M.
PS: I just reread your update, and reread the part about your son not smiling as much:
Look in the mirror and SMILE! he will mirror you -when he sees you smiling, he will know everything is okay.
I noticed when my son wakes and I am tired and just not showing any emotion, he is generally more upset. When he is starting to stir and he sees me grinning like a crazy woman, he just lights up and starts laughing! I try so hard to remember that smiling is something I need to do more often, no matter how tired I am.
Good luck!