I'd add that it may be perfectly normal for him--maybe stocking up on sleep in preparation for a big grown spurt or developmental change, etc., but if you're worried, check things like: diapers, weight change etc. Is he wetting the same number of diapers and the same amount of wetness? Is his growth/weight consistent or increasing? If so, yay!
Otherwise, I'd wonder if you're right next to him during this sleep? I remember my DD in the early months was getting a large chunk of sleep with DH at night. She would only sleep upright on our chests and I had a much more difficult time learning to do that so DH took a big night shift for me. At first, he would wake me up whenever she fussed so she could BF. After a few weeks, she would fuss a little and if DH was holding her and I wasn't right there, he'd pat her a little and she'd go back to sleep and sleep MUCH more than if I were right there and she could smell/access BF. She was getting REALLY long stretches of sleep. We mentioned it to our ped. and he thought it was unusual but GREAT (isn't it what everyone wants, right?!) But after a while, we noticed her morning diapers were dry (which was odd) and more concentrated looking/smelling. After a couple weeks we had a slight dip in weight (from 75 to 50 percentile). We removed DH from the nighttime routine and she started BF more in the night and diapers, weight returned to normal. At 4.5 yrs now, she's been consistently at 75% weight throughout her life except for those few weeks where she spent many night hours with DH instead of me. I know it's heretical to some to say that a long stretch is NOT ok, but in our case, it wasn't and we were glad when DD went back to more frequent night nursing.