Pets will eat grass when they are either bored or have an upset stomach. By eating the grass it will tickle their throat on the way down and usually cause them to vomit up whatever is upsetting their stomach. If Riley isn't vomiting or has a decreased appetite, then he is likely just bored. I would however refrain him from eating grass, to prevent him from having any vomiting episodes.
Thanks for visiting Petco Pet Education Center, formerly Petcoach, and I hope that Riley continues to do well. Best wishes and take care.