Best MX tires for Med-Intermediate Terrain

Hello, I'm looking for some advise on Motocross tires. My son rides on a loose dirt track with some rocks, I think it would be classified as a Medium to Intermediate terrain. He was riding a KTM 65 last year and I would rate his skill level as intermediate. I got him a 2014 KTM 85 this year but it needs some new tires. I know its a trade off between performance grip and tire durability but I would like to get some feedback based on experience which tires offer the best balance of Performance & Durability. He only rides a couple times a month because we live in the city and need to drive to a track but when we do go to the track he usually rides for 6-8hrs. The KTM tires sizes are 17/110-17 Front and 90/10-14 rear.

I figured the rear tire see the most abuse so I was thinking about a Harder tire for the Rear and perhaps a medium tire for the front (advise on this thought is also appreciated). As well as what brand/Model of tire you would recommend based on your experience. I greatly appreciate any feedback anyone can offer!