sparklemagic,
I'm not a great fan of a lot of pre school swim classes. Although there are some excellent ones around, many seem too regimented and not enough fun at that age and many (but not all) children struggle to concentrate in a formal way for 30mins. Your dh sounds like he's doing a great job and your ds could easily continue to learn a lot essentially just splashing about with dad (and doubtless getting more 1-1 attention than he would in a class). I've added a few pointers below to try and pursuade you that swimming teaching isn't all that difficult - especially with a confident youngster not afraid of the water. I guess the time for lessons is when he stops learning from dh.
The crocodiling along in shallow water is actually a recognised way of learning to swim - as your kick gets stronger your hands touch the bottom less, eventually paddling instead of pushing on the bottom.
Swim teachers would be aiming to:
o encourage confidence in moving around in the water - forwards, back, sideways, twisting, jumping etc.
o practise floating and regaining standing position.
o get confidence in putting face underwater & breathing skills - practise blowing bubbles, pick thigs from the bottom, turn face to side to breathe in and do a series of breathing in and blowing out under-water.
o practise kicking - easiest is alternating front crawl/back crawl type kick - try holding side or dad and then with a float. Practise on front (float overhead) and on back (start with float on chest, then overhead). Aim for a horizontal body position and kicking from the hips with flexible ankles rather than from the knees. If the legs sink, then the kick isn't strong enough, use a float and practise more.
o encourage push-glides for streamlining (front and back), from wall to you, gradually increase distance, eventually add kick.
o practise arm movements. To get feet off bottom anything goes - dog paddle is acceptable! Then work towards proper front and back crawl arms.
o breastroke is a little trickier - you might want lessons for that.