I've been there sometime ago in April, the beach is OK and very close to the club. But there is no direct flight from Phuket to HK. There is a mini/petit club for the kid and plenty of included food and drinks! My girl loved the petit club very much although its just some arts and craft stuff!
Infact, it comes in a package, you can check the price in the website:
http://www.clubmed.com.hk/package_rate/02.php
As you can see, 2 days will be travelling, that leaves you with 3 days for play only. For your kid, you can try bringing him to the swimming pool or beach in the mornings and join the Petit Club for activities in the afternoon. It will be very relaxed and not too tired for the kid. There should be other kids in the petit club too (depends) and your son my like to play with them. If you can leave him there, u 2 have some time of your own.
Overall, my daughter like the trip esp. the beach very much! As parents we felt quite relaxed too! The only thing is the indirect flight from Phuket back to HK!
I have been to Clubmed Phuket, Cheriting Beach, Bali, the atmosphere is very good, since the GOs there are very nice, and there are always a lot of activities e.g. shows at night. I took my daugther there when she was 2-3 years old and she enjoyed the stays very much. But the beach is not always good, I think only the one in Phuket is OK (or maybe I went there at the wrong time!) You can give it a try!
Besides, Cebu in Philippines is also a good choice since there are direct flights and the hotel there is very nice (I think is Shangrila Mactan). There are a lot of fishes in the hotel's beach and you can ask the staff to give you some old bread to feed them!