If it's a weight issue, the NHS dieticians are excellent, and provide various supplements free of charge (my husband is in a constant struggle to maintain his weight, so we have lots of experience). Do ask for a referral.
If it's muscle he wants to regain, then exercise is the only answer, combined with a high protein diet (the dietician will advise on this too). Could he get advice from a physio? Mine - who is private, so it may be different - signposted me to a number of suitable trainers and classes following a lengthy period of limited activity.