Tease him a little. If he's not hard enough, pull off of him and make him re-enter. Keep doing that until he's as hard as you want him to be.
Also, if he gets hard while you're going down on him, tell him you want him inside you. Right now. You didn't give him any kind of guarantee that you were just going to suck him off, did you? If a little oralizing is what it takes to get him hard, then use it.
Information on this site is provided for informational purposes only.
It is not meant to substitute for medical advice provided by your physician or other medical professional.
If you have or suspect that you have a medical problem, promptly contact your physician or health care provider.