He is a very handsome puppy! I adopted my first Frenchie several months ago and looked at about a half dozen litters and took pictures of many of them. There was so much variability in them, I am no expert but by the pictures it looks like he could be 100% frenchie. At 13 weeks, it is very difficult to tell by just looking at them as so many changes can take place as they mature. Unless you plan to breed him to get 100% french bulldog puppies, I really wouldn't be concerned if there are hints of another breed in him whether from his parents or even farther back. I can tell you that he looks to have a slightly longer nose than some frenchies but well within the norm that I have seen. From my understanding, this is a good indication that he may be less prone breathing difficulties which is certainly a positive thing.
Did the breeder you purchased him from have the papers of the parents to prove his origins? Unfortunately, once the transaction is complete it may be difficult to recover any of your investment in him. I don't think it would hurt (although it may be too late) to ask the breeder for proof of his heritage if they were advertising him as a full blooded French Bulldog. Explain to the breeder that people that have seen him have raised questions about his origin and if they can't provide proof, it may open the door to negotiations for a partial refund. Having no idea what you paid or your location, I can tell you I have seen brindle male puppies with registered parents go for a minimum of $1800 and up.
I would't let his origin concern you for any reasons other than breeding or over the price you invested in him. Even If not 100% french bulldog, it definitely appears that the majority of his heritage is. There is no question that he should to share the personality traits of this breed that we all love!