You need to learn the particular things that your guy likes. Fortunately, many people who make love to men enjoy fellatio, and they have preferences too. To avoid gagging or sore necks, every couple needs to discover the most comfortable positions. This will often be mouth on top and penis on bottom, allowing the top to control the depth and speed of penile thrusting. Some people like to pretend that they're being forced to fellate, while others hate to have their hair pulled.
Communication is essential.
The person doing the fellating should get the only vote on these questions. You should be in charge of what goes in your mouth or down your throat. Any guy who says he feels rejected if he can't come in your mouth or if you won't swallow his semen is either spoiled or selfish. Maybe we should forget the head altogether. You can invite him to lick his hand after he masturbates to give him some perspective, but remember, you don't need a good reason to set limits.