he might have to hear it from someone other than you. In my experience dads show their love by being providers, maybe he feels the need to buy his son’s affection. Try putting emphasis on what a good dad he already is, spoiling the kid won’t do him any good. If that doesn’t work maybe he can atleast do the shopping with his son when he gets here for the weekend so the anxiety is limited and the son sees all the effort dad goes through. Just a guess, Good luck!
Your husband is soaked through with guilt and insecurity. Other than keeping him too busy to go through his routine, you won't get him to be less nervous. You can offer reassurance and help him channel his anxiety into other things. He wants to show SC he loves them, and right now, he is trying to bribe and buy that love. Help him plan other ways he can show his love. Are there things SC has reacted to positively or mentioned? Many men find it hard to express their feelings, so helping them spend quality time together is a better bet. I also think it's worth getting all the candy together and saying "this is what your kid is eating this weekend." Like, clear off the whole counter and take a picture of it. Seeing it all together can make it more apparent that that's not love. I bet SC's so constipated! Ask him to go to a divorced dad's group. That will help the most! Make sure it's legit before recommending. If he will go, sign him up and make sure he attends his first several sessions.
Sounds like he's being a pick me parent and is in competition with the mom for his love. But that's not the way to get it that child will never appreciate anything he does for him.
If he has to yell at him for being ungrateful for the hundreds of dollars spent then stop spending the money refill sure but no need to go all out especially if the sentiment is not being appreciated. If all of that stuff is already there maybe just enjoying the weekend with what's ALREADY in the house would be a good try and see how it goes. If he still has to yell at him for being ungrateful then there could be a different issue afoot. SS might just want to spend quality time, take him shopping with you or find other things to do with the stuff you already have at home. Buying them is never the answer my BD does this with our daughter and it doesn't work. Good luck I know it's a tough job being a parent let alone a step parent