Blimey, nightmare - I can imagine how grumpy he gets...
My DS2 is 7 months, and generally naps well during the day, but gets very easily knocked off track by a cold, teething etc and takes a little while to get back into his good sleeping pattern afterwards. Anything like those that could account for the sudden change?
My DS1 was always a shocking sleeper, and I eventually got into the habit of putting him down at the same time every day, twice a day - 9.30 and 12.30, iirc - and went through a little routine with him. I tried quite hard to get hime to sleep - resettling him several times - but if he didn't, I didn't force the issue. Eventually he started to have regular naps at those times - so maybe persevering with something similar will help, but it may take a bit of time.
He's at the age where both of mine sort of came out of themselves - you know, emerged from that newborn feed-sleep-feed-sleep thing and started being a lot more interested in the world, so it may be just that everything is too exciting at the moment. But definitely worth persevering, as sleep is good for babies and mummies! DS1 is now 2.7, and I am dreading him dropping the daytime nap - I still find it such a respite.
Good luck, hope you find a solution, it's v tough when they won't sleep.