how long does he nap when you put him down? does he transition fine through 45min?
if he naps 1.5hrs at least per nap then i think it is doable if you really want to move to 4hr feeds. just thoughti woudl throw out for you that 10-15 min BF at 3 months seems totally normal to me and sounds like a full feed, so if you are just worried because at 3 hrs he does not feed for 20-40 min that is all part of them getting more efficient at BFing...and i woudl consider staying with 3hr EASY if he is otherwise happy on the routine you have
anyway IF you still want to move to feeding every 4 hrs (some feeds 3 1/2 hrs, others 4) but working on shorter A times you coudl do something like this (adjust according to wake times etc)
wake 7am
nap 8-9:30 or so
feed 10:30
nap 11 - 12:30/1pm+
feed 2pm
nap 2:30 - 4
catnap 5:30 - 6
feed 6pm
bath and bedtime routine as usual and in bed between 7-7:30
DF 11pm as usual unless wakes looking for it in which case I woudl feed at waking or move DF earlier to catch him before wakes (might not be able to go 6pm - 11pm)
this is just an idea, obviously if he does some 2hr+ naps then you can push the feeds later
i put te timings i did so that you can feed later in the A time, but after 15 min feed still have 10-20 min for A and windown so not feeding to sleep (it is okay to feed later inthe A cycle you just don't want to create a feed -sleep association if you can avoid it)
about your questions
I think he's waking up fairly tired as he's ready for his first nap only 45 minutes or so after I've got him up for a feed
even if he was waking when you were up, he could still be ready for a nap at 45 min. at 3 months my spirited dd was only up 45 min for first A time and then 45min- 1hr for other A times. 3 months is still very young and 45 mn A ti8mes are VERY common especially for first As as many babies see that first nap as more of an extension on night sleep. they wake and want to eat and hav a little up time before hitting the sack again.
This puts the routine out right from the start of the day too. How do others handle this?
don't think of it as "out of whack" if you can help it. it is not a schedule that you need to adhere to (unless that is what works for your lo). EASY is a flexible routine that changes with each child based on temperment and A times that work for them. if a 3 mo old was only staying up for 30 min max through the day or eating every 1-2 hrs I woudl say to try to push them out to stretch a bit more, but when they are doing reasonable A times and feeding 3+ hourly and sleeping through, they are doing fine and the day can be a bit more fluid - sometimes they get really tired during an A time and want to get to bed 20 min early, if he gives you clear cues at 45min -1hr that he is tired I woudl put him down etc. You woudl be amazed how many times it happens that when you put a lo down "earlier" for a nap/sleep they actually add that time onto the nap as they were not overtired for the sleep. obviously it varies for each child, but i have always found overtiredness is the biggest culprit for routine busting so better to follow their cues and err on the side of getting to bed earlier rather than later. HTH