The best way to have energy is to eat a healthy balanced diet, get plenty of water, sleep and exercise too.
A multi vitamin doesn't hurt.
The fasted way to gain weight is to eat foods that are calorie dense. Foods that are high in fat and sugar will do this. Pastries, cheeses, sauces, fatty meats etc. These foods are terrible for general health though.
There are liquid protein drinks that are sometimes recommended by doctors in certain cases. I think you can buy them (in cans) in the pharmacy aisle--one to check out is called "Ensure" (nutritional supplements). I'm not a doctor, though. The best advice is to ask a doctor or health professional. Good luck with your son!