I found that between months 5 & 6, A times jump quite a bit. DD was doing 2.30 at 5 months, which stretched to 3.20 by 6 months. I definitely think he needs more A time, especially as he is fighting naps and is only catnapping. I think A time for a 6 mo is somewhere around 2.45- 3.5. Keep adding 10-15 minutes every couple of days and holding for a bit. It needs to be done gradually to avoid OT. Hopefully, once his A times are figured out, his naps will also lengthen out. I'm pretty sure these these night wakings are due to over tiredness, hun. Basically, he is not tired enough for his first nap and so does a short nap but gets progressively over tired as the day progresses.
Another thing is, for these babies, half an hour makes a big difference. Even 10 minutes can. So once you figure out his A, it is best to put down within 5-10 minutes of that time. Half an hour earlier and he won't be tired enough and will fight going to sleep and get OT or just do a short nap. Half an hour later and he will be so OT, leading to a 20-30 minute nap. The sleep window is usually quite a small one and it makes things easier to put them down on time.
Hope that helps