First of all communication is the key always talk to your Master so that he knows how you are feeling and what your thoughts are,, by doing so this can help you grow as his sub as well...if you don't want to try to swallow first thing then try something different such as him cumming on your chest or something so that you can have a small taste. Also the pre-cum will give you a little bit of a taste of what it may be like or you can try a facial or something like that as well. Take baby steps and when you are comfortable you will know.