I think as they grow up together more, they should be fine. It's all about training, socialization, patience and just time. Mastiff's are so super strong but usually are pretty gentle (from what I've read) when they get older. Your son is just going to have to be sure to socialize, socialize, socialize, and train, train, train!
I think it's wonderful that the Mastiff is a puppy so he can grow up with your little one. I dogsit a Rottweiler and we introduced him and Jackson when the Rottie was around 14 weeks old. It's wonderful socialization for the both of them. I also dogsit my aunt's Pit Bull and her Siberian Husky, as well as my grandma's huge Lab. They've all been socialized with dogs of all sizes their whole lives so I trust them. Of course, I watch them more carefully and supervise more than I would with two little ones.
Your Biewer and the Mastiff are just being puppies, so I think you are totally correct in seperating them, as I'm sure the Mastiff does not know his strength yet. You are right in knowing he can injure your little one. I think you're doing everything right
