Right, where was I.
He doesn't have to have a regular napping pattern, or a Gina Ford-style schedule with naps at set times. However, two or three naps a day is not enough at his age, not when he's a frequent night-waker. It means he's staying awake far longer than he should at his age, which means that when he really needs to go to sleep, he's overtired. This leads to 1) an inability to fall asleep without a great deal of help 2) an inability to stay asleep 3) screaming, clinginess, more screaming, then more screaming or 4) all of the above.
Again, there are babies who don't nap much during the day, but their night sleep tends to be more consolidated.
To re-iterate, all you need to do is watch awake time. If he wakes up at 6am, start watching him like a hawk at 7:30am. First eye-rub, first yawn, you get him asleep. If you catch these first sleepy signs, this is your best chance of getting him to fall asleep, and this first nap is probably the best and easiest chance you have of getting a cot nap if you want to keep trying them. Cot naps get harder to achieve as the day wears on.
When he wakes up, note the time. Go out, but organise your activities so he'll be in the sling with you walking 90 minutes later. Or, do stuff around the house, then plan to stick him in the sling and go out at the first eye-rub/yawn after 90 minutes.
Note his next wake up time. Have lunch. Organise nap-snacks - I used breastfeeding to justify afternoon nap cupcakes purchased during the second nap's outing 
Once 90 minutes have passed, boil the kettle while you watch for sleepy signs. First yawn/eye-rub, take 
and 
through to living room, place in handy table, put wildly addictive trash TV, stick him on the boob and let him sleep. This usually took me through to about 2:30pm when DS2 was four months (I was still up the wall and railing when DS1 was the same age).
Cook, do jobs, go out again, final very brief nap - usually only lasts about 20 minutes - 90 minutes after the third one ended. DS2 usually had this one in the sling while I cooked dinner.
This should get him to a point where he is awake and civilised when your DH gets home. Bed time is, you guessed it, 90 minutes after the fourth nap ended, which should definitely be after 6pm, so get your DH to bathe him and get him into his pyjamas 60 minutes after the fourth nap ended. Maybe even get him to do something like baby massage, just your DS and DH, in the room that he will (eventually...) sleep in by himself so he gets positive sleepy associations with the room - keep the lighting very muted, like a floor lamp only, to start his melatonin naturally flowing.
Awake time extends to 2 hours at around six months (so number of naps goes to three); 3-3.5 hours around 9 months (number of naps drops to two). They drop down to one nap between the ages of 12 and 18 months, and that transition is a can of bullshit I wouldn't wish on my worst enemy.
I know it seems like a lot of work, but babies are hard, hard work that sometimes can't be delegated to others. All of the above may make for pleasanter awake times and by reducing his over tiredness, may have a knock-on to improve night sleep. Or it may not. But it's just how he's made if it doesn't.
Hang in there 
He'll be an elf in the school Christmas play before you know it and all this will be a distant, hazy memory.