Send him up with another CFI. Not being rude or saying anything about your skills. Just with another CFI he won't feel comfortable which I think is the issue. I've come across this.

As a cfi you are always doing little things here and there. Either you don't realize you are doing them or they are so little you do them and don't say anything. In my case I'm always doing them on purporse but don't say anything because I'm already correcting him enough and there is only so much I can correct him on before he'd feel like I'm beating him like a borrowed mule and he won't learn anything at all. The student knows what you do and don't do. Most likely on a subconsious level. Send him up with another CFI. Good cop/bad cop style. Have the other CFI tell him he isn't doing anything for the entire flight except scanning for aircraft. Your student will know he is on his own and will do much better. I personally always flew my best on checkrides because of that. however my orals have always sucked which is why i'm panicing about regional interviews lol. Just my .02

Also positive reinforcement is good to help a student learn. Once you know he knows the materials I find it very effective to come down on him. However do it using terms like "I know you can do this I've seen you do it before but this isn't going to cut it". Serously it makes them do better. While I've never taught an instrument student I've used it on ALL my airforce peeps and I haven't had one fail yet after 38 students