Natalie, it sounds ok to me. All babies are different with their sleep needs. My first DS did not need, or have, a lot of day sleep after about 12 weeks. DS2 can (and mostly does) sleep for 4 hours during the day and 12 hours at night even at 21 months! At 13 weeks he was a lot like you describe with your DS.

I have noticed that DS2 has reached milestones a bit later than DS1, and took a bit longer to be talking. I put this partly down to him being number 2 and having less dedicated time with me, and partly down to him sleeping more during the day and getting less stimulation/learning time. But he has caught up and is doing really well for his age, so there is no harm done. It is just one of the many things that is different between the boys.