The grass won’t hurt him but it isn’t something they normally eat. Is it possible he’s picking up some grass while chomping down on the other plants? I assume the clover is growing in the same lawn.
Mainly they eat broad leaf plants and weeds, so the clover would be one of those food types. My Russian tortoises will eat grass if it's very young shoots, but don't eat the older, established grasses. If they do eat it, it's not a cause for concern.