It's very painful and the ulcers/blisters are in their throat as well, which is probably why he doesn't want to drink or eat. Really cold things feel the best because of the ulcers, so ask your ped what you can give your son at this point - maybe try and make his formula/breast milk super cold by putting it in the freezer for a bit...? The entire cycle of the cocksackie virus lasts 7 - 10 days, including the incubation period when you didn't even know he had it, so he really is probably almost through with it... poor baby...
as far as his being contagious - they are not contagious before the blisters/ulcers appear, only once they do appear and they stay contagious until they are gone... and, yes, you can get it again, unfortunately...