Does he pull back or run around the tunnel?
Following these "Tunnel Training Tips" will have your dog charging through the tunnel to meet you at the other end.
The article gives a step-by-step guide for training tunnels that helps even the most fearful or jittery dog to learn entering straight tunnels, curved tunnels and agility shoots.
I followed these instructions with my own dog and the tunnel is one of his favorite obstacles now. Good Luck!