Incorporate high calories foods into his diet. Cheese, milks, yoghurts, have a google of what foods are high calorie.

My DD was sick earlier this year, and didn't put weight on for months. She was frail, weak and looked ill. She had a Staph infection in her digestive tract.
Since then, she's fluttered between being a good eater and a fussy eater, BUT has put on 3 kg's and now weighs 16kg. Finally, she's moving into the next clothes size. We've spent 9-12 months in size 2 clothes now, bring on the 3's!!

Follow your instinct, do something to his diet if you think it's necessary. xx