I'm a babyswim teacher, so can advise with authority!
20 minutes is the maximum recommended time for tinies, and that's in a water temperature of 32ºC, which is the minimum recommended temperature for babies. Very few pools provide this, so I always recommend a baby wetsuit. There is an online shop where you get a guaranteed part exchange price if you buy a wetsuit from them and upgrade with them to a larger size, and their prices are extremely good as well. www.littlepants.co.uk
Don't dunk the first few visits - make sure your LO is very happy with the water, the situation, and water on his face before you attempt this. When you do try it, first of all show him yourself going under. Then blow sharply in his face first, to make him take a breath in, and then SLOWLY take him right under in a vertical position, holding him in front of you, and go with him, so he can see you under the water with him. If you snatch this bit, you can force water up his nose! If you don't take him under in a vertical position, water can flow into his nose more easily, and although this won't do any harm, it can be uncomfortable!
Most important of all is to smile and laugh when you both come up!
A warm shower afterwards will not only remove chlorine, but also warm him up! Use your normal babywash if you want to remove all traces of chlorine.