nap time grumpiness

So my 8 month old sucks at naps. Every once in a while he will take long one hour to one hour and a half naps. If it's usually under that he will just cry and cry and fuss and cry until his next nap. I've tried putting him back to sleep if he wakes up early but no luck yet. Once he has a crappy nap there goes my day cause he needs constant attention. Not even on the walker will he be happy he just follows me crying. 
Anyone any advice ? Thanks!!