I think olive oil or sweet potato or a Vet prescribed laxative is the answer for constipation. That said eating grass (spray/poison free) never hurt a cat. Plus he seems to love it!
Just watch out for any vomiting. Some cats will gobble up as much grass as they can quickly and throw it right back up Then they go right back to gobbling more grass Maybe let your cat mow the lawn for a few minutes a day before taking it away.