Great advice for the OP ... if you have a dog you think is worthy to breed, post a notice on reliable sites ... Devine Farms, worldmastiffforum, etc ... with nice pictures, testing details, show results (if any), and pedigree.
Also, what does your breeder say? If you got him from a well-regarded line of dogs and your breeder has a good eye, their prospective should be helpful.
Lastly, your original post gave no indication that your vet wouldn't stand to benefit, that you were quite comfortable with the nuances of the breed standard, or that you had sought out breed experts to give their prospective.
Related to this, I only decided to breed my bitch after completing her testing (hips, elbows, eyes, etc) and after taking Winners Bitch at both Bucks and Trenton ... and ... having other hands-on evaluations from Mastiff-specialist breeder-judges ... and after this, she was bred to the most proven Mastiff stud dog on the planet (CH Greiner Hall Amir Zahar of Jadem); at the time of the breeding he only had 14 kids older than 2yrs of age and 12 of them were champions ... and two of his kids have, between them, won Best of Breed or Best of Opposite Sex at the last FOUR MCOA National Specialty shows (one of the dogs is Oliver, shown above, and the other is Moose). And on top of that, their pedigrees were very complimentary ...
Breed the best to the best ... it's easy to breed second best to second best. I have high standards and high expectations and am disinclined to apologize for either one.
I sincerely wish you well ... it wasn't that long ago I was wondering if I should stud out my boy ... and as I learned more, the more I decided I would not, despite being asked by several bitch owners of the could breed to him.
while I don't know your dog, I do know that not every dog needs to be a part of the next generation ... no matter how much the dog is loved and no matter how much friends say how great he is.