What lovely progress your DC has made - well done!
You can get little toys to blow bubbles against, they flip over when you blow them right - they look like little flying saucers with seals heads on (if I remember correctly) and are different colours on the two sides. My DDs swimming teachers also use a lot of nursery rhymes and songs in the lessons, so they would do "Ring a Ring o Roses", and instead of "we all fall down", would do "we all blow bubbles". They would alternate this with doing "we all fall down", at which we would pull the children under the water and pop them back up again - which DD LOVED!!!!!
Star floats - she started by having her head on my shoulder and lying out in front of me, so she knew she was supported. This was done singing "Twinkle Twinkle Little Star", which distracted and sort of limited the time it was done for. It took a while for her to learn to stretch her arms and legs out flat, and she would flip over a bit at first, because I think it is quite unnerving to learn to be still in the water and trust you won't sink. Telling her to see how still she can be, and for how long seems to work quite well too, once she had the basic technique.
If you are teaching him to kick, it might be good to have a float vest, because it will hold him up, and (I'm assuming he is wearing trunks, so no fabric to grab round the middle of his body) give you something central to hold onto while he concentrates on the kicking! They have just got a load of float vests in our local discount store - I'm gutted as I got mine for ££ from Mothercare! You could show him how you kick your own feet, really exaggerating the splashiness, our DD was told "splash your feet", and praised for really making a big splash. Again it took a while for her to get this. If you have anyone who can hold him up in the water, while you manipulate his legs, this could help him get the idea of what it feels like; alternatively, he could hold onto the edge of the pool, while you do this. At my DDs lessons they do things like putting lots of little toys on the side, and a big float at the other end of the pool - the children pick toys to take from the side to the float, the more they kick, the faster they will go, and the more toys they will move in the time. Or moving little boats from one side of the pool to the other, putting ducks from the side into buckets on the other side, "Posting" floats into the water vent thing (probably not in a public pool - they might not like it :)) - or any permutation of this kind of game you like. Once the kicking is established, you can work on synchronising the arms...
My DD loved learning how to move round the edge of the pool with her hands, quite good for safety too, as if they can get to the side, they can "walk" their way round to the ladder even if they aren't able to get out safely yet.
And you will also want to teach him to get out safely. Holding on to the side with both hands, put one bent arm right up onto the edge of the pool. Help him push/pull one bent leg onto the side too, from which he can hopefully learn to pull himself up onto the side. (Trying to think how they taught my DD, and I think this was it - hard to imagine it away from the water). Had to give her a boost up at first, but she soon got it.
I hope this helps give you a few ideas. I'm no swimming teacher, but my DD has been going to lessons for over 2 years, she is now 3.10. I did go in with her until she turned 3, so have "hands on" experience of following the teacher's instructions, for what that's worth (possibly not much!!!!!! :) )
The Zoggs website has some good toys and the flippy blowy things here:
shop.zoggs.com/productlist.asp?A=ResetSearch&CatID=13&RootCatID=12
Have fun!