What you should do is take it easy at first
There is a lot here for you to read
Foreplay is the key >> https://www.zoovilleforum.net/threa...-please-a-male-dog-a-guide-to-oral-sex.21551/
Some tips
NEVER, EVER pull on his tail. If you have to hold on, grab a leg.
Start small and work up. A 50 lb dog is sexually equivalent to a 200 lb human.
DO NOT handle a dog's bare penis. It is so sensitive that the fingerprint ridges on your hands are abrasive to him.
Be careful not to "help" him too much or he will expect you to do all the work from now on.
Long coated dogs will last longer and be more active in cold weather (natural or artificial).
Remember that you dish in where bitches bulge out. Stay a little lower or to one side of him while tied so you don't trap and squeeze his nuts between you when he is turned.
Security is important to dogs too. Do not be surprised if he doesn't immediately want to perform in a strange place, with strange people, or with other dogs around.
Pet him and praise him during and after the act. Dogs have an ego too.
Use any lube that suits you when stretching. J-Lube is my favorite. Use only water based lubes without additives or spit when he is involved.
DO NOT get excited if he pulls his knot out and you feel wind blowing inside your ass. While it is possible to tear something, this only happens if you manage to do something spectacularly stupid and difficult. A little judgment and preplanning will go a long way. Even if you make a mistake, you will stretch a LOT before you tear. And it may take some time for the stretched muscles to return to normal,but you will return to normal. Stories of dribbling shit for the rest of your life are just that, stories.