How about a second level with a ramp?

How hot does it get where you live? You might consider doing something to keep him cool in the summer because bunnies cope with cold better than heat.

Is the bottom of the cage solid? 

My bunnies like granite tiles to lay on in the summer- even though they are inside and we have air conditioning.  They like to lay on the hard cool surface as opposed to the fleece that lines the floor of their pens.

You could also grow trays of pet grass in a shallow container- and then let him have the whole container to eat the grass and dig in the dirt.

I think maybe I would cover one of the sides, so that he is protected from the wind and can feel that he has his back covered from wild animals etc. It gives the rabbits a feeling of security. That is why you often see them sitting in a corner.

How about adding some large branches also?

I think the idea of a container with dirt and grass is excellent