As with many other sexual activities, there is a physical sensation aspect and an emotional aspect.
Physically, it's a bit like having a warm banana in your mouth (can you do so without leaving teeth marks?) The taste can vary from barely noticeable to 'you need to wash before I am going near it again' but is fairly distinctive.
Emotionally, you've got his penis in your mouth!?! There are not many things that are more intimate.
You can ask for guidance as to what sort of things feel good for him and he may ejaculate as a result - if you don't want him to do so in your mouth, you need to tell him - or he may not. It may happen quickly or not at all - giving oral sex for a long time can result in an aching jaw if you're not used to it, and if it's not fine to take a break, you need a new boyfriend.