How much sleep is normal for a 10 month old?

My LO just turned 10 months and the last 2 weeks he has been napping longer so he sleeps about 15 hours on average in a 24 hour period lately. I'm concerned that 15 hours is too much? He used to sleep 13-14 hours before this sudden change. Is this a normal amount? He isn't sick. Could it be due to a growth spurt? The only other thing I can think of is that he is getting 4 teeth coming in but I would assume that would make him sleep less? FTM here so I'm clueless and not sure if I should be concerned?