Really good ideas there by Reality chick - other foods/ideas you might try:
Add powdered milk or cream to his milk (about 1tbs wholemilk powder and 200ml wholemilk - adds 50 calories plus protein) or 2 tbs double cream adds 60 calories
Give him hot chocolate (2 tbs hot choc powder, 200ml milk, 1tbs fullfat milk powder)
Fried bread - 1tbs oil or butter and 1 slice of bread - 190 calories - serve with ketchup and or scrambled eggs
Put a heaping tsp of butter on his vegetables/mash, or a drizzle of olive oil, at every meal.
Greek yoghurt with banana and raisins
Peanut butter or any other nut butter, smooth, stirred in to hot cereal, on toast, with apple slices, spread on banana.
Try to give him something small to eat/calorific drink every hour or two between meals. He might not eat as much at meals but the calories during the day should make up for it.
My sister and I had (and I still have) problems with my weight due to a condition, but my mum would feed us up with snacks throughout play. We'd typically have breakfast of cereal and 'special' milk at 7, toast with butter and marmite at 9, a biscuit and more milk or juice at 11, lunch of cheese sarnies and crisps and salady bits, a yoghurt or two at 2, a cracker and cheese at late afternoon, Dinner, then a chocolate bar a bit later. More special milk before bed. It looks loads for a small person to eat but the key was the small portions and high fat. I don't remember specifically ever being hungry as such but I certainly never felt like I was being force fed. 
Really hope you get some luck with his weight soon. He has been tested for anything which would cause his low weight hasn't he? xxx