We have a full lifejacket that I bought for my DD to wear at the beach when we went to Mexico. She loves to play at the edge of the waves, and I'm always worried she will trip and get swept in by mistake. We bought it at Sportsman's Wearhouse, although you can get them anywhere boating equipment it sold. (some REI stores, some sporting goods stores). I would only get one that is approved for boating. They are rather hot and bulky, but worth it if you are going to be doing any beach play or water sports. They should fit snug to the body, so that if you grab under each shoulder strap and lift, you can lift your child in the jacket without the jacket slipping up over his chin. When wearing the jacket, you have to have all straps fastened, including the one between the legs, for it to be safe. Pay attention to the weight ratings, not just size. We were very happy to have it as we also took some boat taxis and there were no lifejackets for anybody in evidence. I can swim, as can DH, but I wouldn't trust myself to keep DD and myself afloat in deep water for any length of time.
If the villa you are renting is in the US, it should have a fence with a locking gate around it. If it doesn't, you will need to figure out a way to lock doors or put in gates so that he never has unsupervised access to the pool. When in the pool with DS, you can have some toys to play with but the best thing really is just always being in arm's reach so that there is never a chance for drowning. Just always be ready to go in the pool too when DS wants to swim. Enjoy your holiday!