Just wanted to let you know that my ds sounds a lot like your lo. He is now 4.5 months and, as of the past week, has finally settled into a routine. He would do 45 min-1hr. 'cycles' in the morning of awake, asleep, awake, asleep. Then he'd sleep three hours in the afternoon. I never woke him because I just couldn't find it in myself to not appreciate these long sleep periods
He did start on solids at 4 months. He has reflux and it has settled his stomach. But I also think it has made him more satisfied. I would have the same issues with him waking and not necessarily looking tired but I knew he was because he couldn't stay awake even an hour. Then he got way overtired.
Other than solids, I think he was just a little behind in developing his pattern. The way I tried to be consistent with his feedings was to never feed him unless it had been three hours. If it ended up the feeding was right before a nap, I'd be sure I at least changed his diaper or something after the feed to separate it from his nap. And I knew he'd have that long stretch in the afternoon where feedings would be 4 hours apart.
My boy can't stay awake for more than 1.5 hours at any time of the day except for an occasional evening. It is difficult to judge his tired signs so I do my best to keep him up this full 1.5 hours. I found that helped him sleep longer---I was so worried I'd miss his tired signs that I think I'd put him down too early.
Finally, the older our los get, the more they tired themselves out with their physical work. I think all these things worked together to balance him out. Since he can't stay awake more than the 1.5 hours, even a 2 hour nap doesn't get him to the next feeding but I just wait 1/2 hour to give it to him when he wakes up.
I'd just suggest to keep being sensitive to when he's tired, etc., and things should even themselves out. I agree with the post that states it's not a good idea to feed them right when they wake up. It could lead to snacking as well as their wanting food at the end of every sleep cycle. I know there is the idea of extending 45 minute naps, but I guess I always assumed that since my boy would take a nice afternoon nap and sleep well all night long that wasn't the issue. But i don't know if it may be for you or not.
Best of luck---it only gets better!