Have you tried different positions? Doggie, missionary, on your sides spoon fashion? I found that with some guys, the spoon position made him seem tighter to me, which may be what you're lacking. So experiment, see what is snuggest with him. And don't cum for at least a few hours before attempting anal, so you're fully "charged," plus avoid alcohol and drugs.
Also, if you're well endowed, try a larger (XL) condom. And put some lube (approved as safe for condoms) on your dick head before putting the condom on, and keep your foreskin retracted as you roll it down the shaft. Don't get lube all over the shaft, however, especially the base, or you may find the condom slipping off.
What DOES makes you cum? Oral, hand (his or yours)?