Breastfeeding knowledge has moved on a lot since Tracy wrote the books and as she is no longer here to update the routines we try and share the info on the boards instead.
His weight and height centiles are just fine - there's no issue with where he is, they would only be concerned if he dropped more than 2 centile brackets really. Hopefully he has stayed the same, but the fact that he has stayed the same for the last 5 months is a good indication he is getting enough milk!
I would just do a top up feed before a nap for now, if you plan to keep breastfeeding for a while yet that will help your supply anyway. It is a fine line between OT and UT and as they get older sleepy cues get less reliable. They do get used to going to sleep at certain times and do need us to help them push through it, cranky can be bored sometimes too! Just extend his a times by 10 mins for the next few days, and then 10 mins again. If he short naps don't cut the next A time by too much as that can just create another UT nap.
You can just go back to a 3 hour feed routine, but I would work on extending the naps as they will have an impact on night sleep too.
Laura