Hello, and welcome to WP.
I'm a fifteen-year-old female with AS, and you're dealing with a seventeen-year-old male, so my advice might be of some relevance, but I can't be certain. Also, everyone with AS is different (which I'm sure you have heard before.) Not just from neurotypicals, but from each other, too.
As far as the "shutdowns" go (not speaking with people; not really wanting to interact, feeling depressed... etc.) I have found that just being left alone for some time really helps me. The more adults (or peers) try to pressure me into interacting, the deeper I sink into myself and the worse I feel. I'm not sure if this will work with the specific young man you are trying to help, but it does with me. I would consider trying it (if you have not already.) I know it doesn't really sound like much, and that's because I'm never the one trying to get someone with an ASD to communicate.
The eating thing is relevant to me, too, but that's just because I have sensory problems. I only eat a select group of food... I'm very strict with what I eat and what I don't and I rarely try new foods. Has he stopped eating since his mother was diagnosed, or has this been an ongoing problem for most of his life?
I hope you find some answers here.