You can start taking him to the groomer once he's had all of his shots. The earlier you start, the easier it is for them to get used to it. If his hair isn't too long, you can ask for a shampoo & a little clean up/trim. Be sure to tell the groomer it's his first time so he/she knows to be gentle & patient
Also, it REALLY helps to speak with the groomer to let them know how you'd like (or not like) the hair cut. You don't want to be shocked when you pick him up. Good luck!!