My dd, 7 months, always used to nap on me, in the pram, or in the car. The naps on me were a conscious decision, as I didn't want her to be a catnapper like her big sister (30 mins x 3 times a day, exhausting). So sleeping on me meant I could pat her through any short naps, if you see what I mean.
But eventually, at about 5 and a half months, I thought, this has got to stop... Partly because I am going back to work, partly because dd2 was getting disturbed by whatever was going on in the room, and partly because I needed the nap time for dd1, myself, the house, etc!
So, I cleared the diary for a week. At nap time, so 2 hours after waking for the day, 3 hours after waking from first nap etc, I went to our room, put her in her bag, put the blinds up, fed her, popped her down drowsy.nstayed with her til she went to sleep. Because she was tired but not over tired, she grizzled a bit, cried a little, but went to sleep after a bit. First time, 30 mins to get to sleep for a 30 min nap. Second time, 25 to get to sleep, 45 min nap. I should point out this wasn't 30m of crying, it was mostly her gurgling, grumbling ect
She now prefers her cot for naps, will go in, squeaking a bit sometimes, sometimes not, and will sleep for 30 m up to 3 hours (those are good days indeed).
I guess what I am saying, and what I believe, is that babies love routine, and consistency. I would concentrate on getting him to nap in cot in first instance, however long he stays asleep for, then work on the nap length. I got extremely anxious with dd1 because I thought her brain wouldn't develop properly because of the cat naps. Yeah....
And make sure you really really do know his tired cues. Sometimes it's easier to go by the clock... Pop him down 90 mins after he wakes tomorrow and see what happens! Good luck!