First off, you need to find a way to tell him. It would be more hurtful to let him go on thinking you like what he is doing when you don't. Eventually it will come out so why bother wasting both of your time on something that isn't working? To find out months later that you have been turning off your partner rather than turning them on is far more hurtful than some loving advice and honesty.
He is doing it to please you so be encouraging and making suggestions would work best. If there is something he does that is good or at least acceptable ask him to do it more.
His last gf may have liked whatever he is doing, so he thinks that works for everyone. Or perhaps he just doesn't have any experience. There are books on the subject if he would be receptive to that.
If he is super sensitive to constructive advice, then you can get a book on sex that includes oral. Say, I got this book and thought we could try some new things. That way you can help him with his skills and find some other fun things to do together.