It helps to give a treat after testing, and go with a '3 tries and stop' rule.
You also may give treats when doing related behaviors - ex. Rub the ears while petting him & give a treat. This helps shape and reinforce the desired behavior for testing.
A dab of Neosporin with pain relief may be applied to the ear a few minutes before testing and wiped off well to test. It helps reduce the annoyance of testing and helps the blood bead up.
In an urgent situation, such as low numbers, you can aim directly for the vein. Just be prepared for a gusher, get some on a clean fingernail, and blot before he shakes it off his ear.