Just take it slow. Loads of foreplay always helps. The hornier he is, the more he'll want it which will make it easier in the end. Loosen him up as much as you can with your fingers/tongue/toys first, making sure to use plenty of lube. I've always found that teasing helps. Work him up till he's gagging for it, then start again. Do that a few times until he's begging for you to enter him, then do it some more. Chances are it will hurt the first time, but if you do it right, and give him time to adjust, it won't hurt for long and you'll both enjoy it.