hi Ali
A time is from when he wakes to when he goes back to sleep. If this has not been made clear to you then this is why he's getting overtired as he's up for too long!
So if he wakes at 8am, once he's dressed and you have breakfast I would bring that 9.15 feed forward slightly to 9am so you can have him back in bed at 9.30-9.45ish.
Obivously all babies are different, but a "typical" routine at this age would look something like this:
8am wake
9.30am nap 2 hours
11.30am wake
1.30pm nap 2 hours
3.30 pm wake
5.30pm catnap 45 mins
6.15pm wake
8pm bed
As he gets older he will be able to stay awake longer and his naps will get slightly shorter. By 6 months, just to give you an idea, he will probably be having 2 x 1.5hr naps and a 30-45 min catnap, with awake times of around 2.5 hours.
Don't get stressed about not being able to read his cues, many babies' cues are hard to read. I have been going by the clock rather than cues since Sophie was about 6 months old!
I used to take Sophie's mobile down for naps until she was about 6 months old; in hindsight I don't know if it was necessary as if she was tired, she went to sleep anyway. I would carry on taking it down until you are comfortable with a new routine and he is napping better.
let us know how you get on
xx