My DD was a nap refuser from a few weeks old. She used to be grumpy and irritable by evening but I managed it somehow as she slept through the night...then 4 month regression hit and she woke 1-2 hourly at night plus didn't sleep all day and was a grumpy and irritable mess the entire day. This was when I used the sleep consultant.
I had read a few baby sleep books when she was a couple of weeks old and tried a 7-7 routine but I always failed at the first hurdle - she never went down for morning nap, let alone anything else after that. I tried at various times most days every possible method of getting her to go to sleep - rocking, swing, feeding to sleep, pushchair etc etc. Between 3-4 months old, I used to walk for 2-3 hours a day with pushchair and she slept for 30 mins in that time - the second I stopped pushing, she woke up. If I walked into a shop, she woke up etc.
I was at my wits end and struggling because she was so irritable, clingy, crying and always wanted to be held so we contacted a sleep consultant. Sleep training her was a godsend - she went from being irritable to being such a happy baby, she gained weight so much quicker and being with her was a delight. I can honestly say I loved my maternity leave after her mood changed.
She is now 2 and if I gave her the option to miss a nap or sleep late, she would grab it with both hands. Biggest change is sleep training taught her that when I want her to go to sleep, its not a negotiation, she has to sleep.
i have a second who is 4 months and I have made him sleep regularly from birth (relentless rocking as soon as he was awake for 1-1.5 hours) and never allowed him to get overtired and I can be so much more relaxed with him. If he's tired, he falls asleep on the move. We are having some difficulties with him and night waking but I have no intention to do anything about it because he sleeps well during the day and doesn't get overtired.
In terms of nap lengths for your DS - sleep consultants promote short morning naps followed by a long lunchtime one. That doesn't mean that's the only way though, I have multiple friends whose babies did long morning naps followed by 30min-1hr naps rest of the day. First nap should be around 2 hours from waking so definite win that you got him down then. I would expect second nap to be 2.5ish hours after waking from first. If he wakes from second nap early and you get him up, I would expect a third nap around 2ish hours after waking, maybe sooner if second nap was really short. A sleep consultant will get you into a fixed routine and go through it all step by step. They will get you to complete logs of feeding and sleeping and discuss them with you so you can see how to improve things. If you are really stuck with sleep (whether its because DS is irritable like my DD used to be or because you are not coping with sleep deprivation), I would recommend it.