I would be honest with him and educate him on supplements. Explain to him that they are there to supplement your diet - not replace a quality diet.
Get him a quality protein powder, fish oil and multivitamin.
If you don't support him on his interests, he is going to go behind your back and buy the supplements on his own. Chances are he's going to buy some gimmicky pre-workouts, or even worse - a pro hormone.
I bought my 16 year old cousin a tub of wheytobuildmuscle (protein powder), ruthless 3d (multi vitamin), and krill (essential fatty acid) from n2bm.com for his 16th birthday because I wanted to support his interest in training.