Urea and creatinine are the two biological parameters used to indicate the extent of kidney damage. Urea changes quicker than creatinine so urea levels generally represent how the kidneys are now. Creatinine levels take longer to respond so generally show more chronic damage.
The parameters are no indication of how well or ill the dog is - you say yourself that he is putting on weight and eating reasonably well. The elevated creatinine shows that there is kidney damage there, which you knew already. It may likely continue to slowly rise as the condition progresses.
The urea isn't high because your dog is currently able to cope with the damage, through the combination of drugs and the renal diet.
There is no way of prediciting how ill your dog will be at which urea/creatinine level. The levels are good for monitoring individual cases and how they progress, but as the normal ranges can vary enormously between dogs you can't really make a judgement based on them alone. As you know, very high is bad! But that's really as far as we take it.
Continue as you're doing with the diet and the medication, and ensure he has access to plenty of water and you'll be giving him the best chance. If you have any doubts like this, you can phone your vet and ask them to clarify things or explain them again. If you talk to the vet who saw your pup he'll be able to give you a better assessment than I can.
Hope this helps!