Amount of night time sleep and bedtime question...

My 3.5 month old had been sleeping 7pm-7am (waking 1-2times) while I was on maternity leave. Now that I'm back to work we have to get him up at 6 to feed and get ready so I started putting him to bed a little earlier, 6-630. He's a good sleeper right now and will often go until 3 or 4am before waking up but the problem now is that he sometimes doesn't want to go back to bed. I've tried using his swing, rocking him, etc and sometimes it works - but then at that point it's 5-530 and we have to get up soon.

I'm thinking that since he goes 9-10 hours straight before eating that he doesn't want to go back to bed even though it's still night time and dark out.. We're trying to put him to bed a little later but sometimes he's really tired by 6 and typically its hard to get him to go down for his last nap of the day (around 4-5ish). We're slowly trying to get him down later but its tough...

How much sleep at this age do you think he needs? Maybe hes ok with 10 hours? (But then sometimes he will sleep 12..)

When do you put your 3-4 month old to bed? Any problems with that last nap of the day?