You need to work his ass. Get him really relaxed, rim him for a bit if you like doing that, then using lots of lube gently ease a finger inside him & play with his ass for 5 minutes or so, then try to slip in a 2nd finger. Keep going gently & then try a third finger. A butt plug is great too, as they are smaller than a dildo & designed specifically for the ass...
Once you have been working his ass for a while, try to gently ease your cock inside him, make sure his ass is really well lubed up & make sure your cock is too. A condom will actually help it to slip in as well (not to mention much safer of course). You just need to take it really slow, you will get (in) there in the end... :wink: