If the 17 year old looks slightly older, he might just pass by through some bouncers, it all depends on the spot. If he asks ID, you'll be out of luck.
Of course, the legal age to drink/purchase alcohol is 18. But if you move to the interior of the island, as previously stated, some won't mind giving drinks to underage people (kind of part of the culture, lol)
Try to have a guide or someone with knowledge of the interior of the island, since a tourist or someone with no familiarity of the island can get lost. Also drive safely, since there are a lot of curves and it gets foggy this time of year in the mountains.
Edit: When I mean interior of the island, I mean in the mountains, some areas near the coastline, like Mayaguez or Ponce are not AS strict as San Juan, strict nonetheless. It just depends on where you go.