It is a very bad idea for teens to take antidepressants unless as a last resort and within 24-hour monitoring as some teens get an extreme adverse reaction from them. As in, some slightly depressed teens feel overwhelmingly suicidal as a side effect of anti-depressant medication.
Do everything you can to help ease his mood. Check that it isn't exacerbated by things like Vit D deficiency, B-complex deficiency etc by adding these supplements to his diet. Vit D spray is more easily absorbed than tablets. Get a B-complex and maybe a herbal-based iron supplement for him (better than standard iron supplements as it doesn't make you constipated as a side effect.)
Encourage him towards activities that help lift his mood, even for a moment, such as funny TV shows, cute animal videos, nature walks or runs, exercise in any form, video gaming so long as it's not addictive/non-stop, meet ups with friends, maybe in your own home - pizza and film nights, easy stuff. Not that these cure depression but they can help distract the mind temporarily and those brief distractions can add up to significant breaks from feeling low.
If he is amenable to it, he could also try online affirmations, visualisations, meditation and CBT/DBT/EFT tapping etc.
He should be able to access 6 online CBT sessions free on NHS via self referral.
EFT techniques can be self taught using youtube videos.
DS2 has depression and I have kept him away from medication as he is still in his teens. he uses all these techniques and they really help him minimise its severity.