It's absolutely normal. Let him sleep as much as he wants. they are expending so much energy at this age that they get tired very easily. At this age, they should be sleeping 14 1/2 hours or more a day. There is nothing wrong with it at all, in fact it should be encouraged. Just make sure that you are feeding him a lot during the time that he is awake. My daughter (15 mo. old) has sleeping patterns very similar to your son's and there are days that she will sleep so much and then when she is up, I am in a rush to run errands and stuff and I will put her to bed at night and start thining back on the day and realize she didn't eat much, but it is because she is sleeping through the normal meal times and then I forget easily to make sure that she eats because she doesn't ever express to me that she is hungry or wants to eat. So let him sleep, but make sure that despite how much he sleeps to make sure that you still get in the 3 meals a day and some snacks and drinks, because when they sleep that much, it is easy to skip meals and stuff accidentally.