Sorry OP didn't realise I was telling you stuff you already knew better than I do
- I am fairly new to MN!
My nearly 3.5 year old has learnt to recognise numbers 1-10 accidentally from a star chart - the aim was to get him to dress himself in the mornings, not to teach him to recognise numbers! I just drew out 10 boxes and wrote the numbers in order in the boxes - he and I agreed together that he would get a star each time he got dressed by himself in the morning without making a fuss or running about naked (nothing wrong with3 year olds running about naked indoors IMO except when it's for half an hour at getting dressed time!). We agreed on his reward - a little toy 'Mater' from the film 'Cars' once he had 10 stars.
He so desperately wanted Mater that he not only dressed himself speedily without being asked every morning for 10 days (miracle) but also was deeply interested in which number star he was sticking on, and asked and taked through the day about which number it had been, and started spotting the numbers out and about as well. The recognition of the numbers seems to have stuck (though he is back to being a pain in the bottom about getting dressed - sigh!
He could admittedly count to 10 by rote before the star chart was introduced but had shown no interest at all in written numbers (or letters except the first letter of his name, which he recognises but could never control a pen well enough to write).
Does your son recognise his first name or first initial written down? Could he pick it out from a group of 3 or 4 name tags etc? If he can then it does suggest the writing is more a motor control issue, which I think will come with time probably, but if he doesn't recognise his name it could be a different issue and worth knowing if there is that side to it too? Still think 4.8 is young though!
I agree about casually bringing in counting and numbers in every day life being helpful, and just thought the star chart (for something else, not drawing his attention to a need to recognise numbers - but counting the stars) could help, esp if he really wants whatever you agree as the "prize"