I think it's fine to switch to one nap this early if he's fighting the second nap and can make it til 11:30. How long have you been on 1 nap? I think it took my dd close to 2 weeks to make the switch. She stayed at exactly 1.5 hrs during that time, then gradually transitioned to 2-3 hrs around 2 or 3 weeks. She also woke up earlier during this transition time (so in total, much less sleep than before), so that was hard, but again I think it only took a couple weeks to iron that out as well.
So I would say if you're in that 2 week timeframe, wait it out a bit longer.
Also, it sounds like you're having more serious nighttime waking issues, which may or may not be related. It's probably due to your baby being overtired. Walking is certainly a big milestone, so it could be that, or teething, ... it always seems like there's a good 10 reasons why they might wake up in the night!!