I don't think you should wrestle with what he should eat if he has a restricted diet. He owns that. What if he needed special medication? Should you supply that as well?

He brings his own clothes, sleeping bag, toiletries and misc. items. If his diet is that restrictive he should bring that as well. If you feel it may be a problem for your son get the future 'father-in-law' involved in the planning putting some burden on him to satisfy everyone including himself.

Most vegetarians with restricted diets don't impose on others to conform to they're needs. If this future 'father-in-law' does you'll have other problems in the future. Better to nip it in the bud.