DS3 is 15.5 weeks (11.5 corrected) but still sleeps an awful lot during the day. I struggle to get him to stay awake longer than 1.5 to 2 hours and if I go out of the house he will drop off in the car/buggy within 5 minutes, even if he hasn't long woken up. He still sleeps at night, it doesn't seem to make much difference to how long he sleeps during the day.
When I raised this with my HV she thought it might be a problem and suggested I tell my GP (I've got to go about something else on Friday). I am a bit over-sensitive about his development because DS1 has ASD so I'm watching him like a hawk and I don't know if I'm just getting paranoid. I am also wondering if sleeping for so long will mean he develops certain skills later than is normal.