I think you just need to keep at it and keep saying and showing that you consider him family. Maybe not use the word "family" as he might be too raw from his mother's death for that (his heart may scream "you're not my family, I don't have any family my mom is gone!") but more along the lines of "our home is your home."
He may not be letting himself break down the boundaries and really integrate into your home as an unconscious defense mechanism. Perhaps by letting himself feel like a guest, it makes the reality and finality of his loss something he can avoid for a bit longer?
When my SD moved in with us when she was 13, she acted like a guest for a long time. And this was someone who was living with her biological father, step-mom and step-brother who she had known for 10 years, and 2 half-brothers. Someone who spent every weekend and many school vacations at our house. But there was something about the finality of moving in with us, of moving away from her mother, that put her into "guest mode." Asking permission to have a snack or waiting to be offered one. Asking if it was OK to take a shower. When I would throw out my weekly "I'm going grocery shopping is there anything special you all want" she would never chime in with what she wanted. Getting a list of things like her favorite cereal, snacks and lunch items was like pulling teeth. She needed clothes and personal care items but wouldn't tell us, even when asked. We looked at the sorry state of her laundry and finally I just had to take her to Target and tell her to pick out underwear, bras, socks, shampoo, deodorant, hair accessories, etc.
For this year, for Christmas, I would get him some gift cards today to stores that are popular with kids his age in your area or Amazon. That way although he hasn't been able to articulate what he would like, there is no denying that you are treating him equal to your children regarding gifting. So if you spent $200 on your kids but only $50 on him, get him $100-$150 in gift cards so that he can order things on-line or go to the mall with friends and pick out what he likes.