Cats butt gets blocked up

I have an 8 yr old boy house cat.
His butt gets blocked up by some fairly large, hard stool.
I took him to the vet a few times when his anus got so large that he was in a lot of discomfort. They knocked him out and cleaned out his pooper and told me his anal glands seemed to be plugged or something.

The recommended giving him pumpkin to assist him.

His diet is Hills prescription Diet X/D [he had a lot of stones in his kidneys about 4 yrs ago] This food used to be called CD OXL, just recently had a name change.

He eventually passes his mega stool, but in the meantime he will puke from time to time.
He may go 4 months without this happening, other times it will happen a few times a month.

Any suggestions as to what to give him?

We actually put some vaseline on a QTIP last night and put it up his butt in hopes we would pass his "boulder" he's had trouble with the last few days.......and yes, we did find his rock hard stool on the living room carpet this morning

Thank you for your assistance with my boy!!!
Scott