not all babies can do 12 hours overnight, so looking at what you have you are getting 11-11.5hours, which is pretty normal, so I'd start with trying to get bedtime closer to 7pm if you want a wake up after 6am & also push out his nap to never before 11am & gradually move to closer to 12noon.
I think at 15mo you really have to work on more a set nap time & keep it at 1 nap, with my DS1 he was on 2 naps until 20months (pretty much related to my work hours) but he pretty much went to bed 8pm & was awake by 6am most mornings, when we got him to 1 nap, he was sleeping 7.30pm to 6/6.30am.
I think with the 4.30am wake up you just have to find ways to keep him going until at least 10.30am, preferably 11am otherwise you set his body clock on 2 naps & that will give a 10 hour night.