My husband had difficulty maintaining weight due to his illness. We used Ensure drinks to help, these also contain very important vitamins. I think Boost is pretty good, also.
As previously stated, you are probably looking for whey protein. Whey protein is a staple for most bodybuilders when trying to put on lean mass, i.e. Muscle.
Reason being, is that the general consensus is if you want to gain lean mass, you need to consume 1-1.5 grams of protein per pound of body weight/ day. So a 180 pounds man would need 180 grams of protein a day.. Thats alot of protein if only consumed in solid food form, but a whey protein supplement gives you ~27 grams of protein per serving. Other recommended sources of protein in your diet should include plenty of fish and skinless chicken breasts.
S&R in Cebu carries a whey protein powder, the same one I use from Costco in the states. I think i remember seeing it priced at 3000-4000 pesos for a 6 pounds bag, with approximately 75 servings (double what i paid in Costco, but still a good deal compared to what i have seen around town).
There is a wealth of information available on YouTube regarding fitness and nutrition. Two of my current favorites are GoKuFlex and Mike Thurston.