Personally, for me - I like wild-swimming, and I will take my son swimming with me in the sea, in rivers + lakes etc.
In fact, we had a great time this summer at a wild-swimming spot in dartmoor. He has been in the sea a lot, but this was his first time in a river. It was great fun.
The other posters are right though, it has risks attached.. but that is the same as a lot of sports and more wild activities. Skiing, Snowboarding, Surfing, etc.
The key for me is are you competent enough to manage the situation until he is able to do so?
Most accidents in the water involve groups of young lads with no supervision who have no idea what they are doing, and have no support/emergency plans in place. Or in the sea, they involve irresponsible activities of - mostly older, occasionally drunk men, who think they know best, even if they have never been to that spot before.
In your situation, at 12 years old, it would be a 100% no, without supervision. With me there, it would be a yes - as long as he is a very competent swimmer, and goes swimming first, before jumping in (always essential...), and that I've tested depths etc. He would have to have me there swimming with him, not just watching in clothes, so that if anything did happen I was on hand to help, and that I've checked the pollution reports/rating for that water.