Hi Matt-
Wow a Presa! Wonderful dogs. Bet he is just lovely. Yes you are correct that giant breeds like Mastiffs can grow up to 24 months of age. 4-5 out of 9 on BCS is perfect and even if he drops down to a very conditioned (well muscled) 3 it is by far better than being even a 6 on BCS. I’d always perfer to see a dog on the thinner side than even slightly overweight.
Given that you got him from a rescue situation and do not have a ped on him its going to be more challenging to determine if he is inline with standard growth and size for Presa. Normally you could use the parents as a gauge, but not knowing his parents you can not do this.
I checked out a Mastiff forum really quick and in the Presa section most peoples opinions are that you should worry less about weight and focus on Body Score, just like you’re doing. Everyone talking on the thread had dogs the same age that had different weights. You will still need to adjust his portions as he fills out, but I would focus more on Body Condition rather than actual weight. It sounds like he is growing slowly which is what you want. Also if he is highly food motivated you should use his kibble for training basic stuff rather than treats. Only use treats when you need something high value for more demanding training like recall commands. I dont know what you intend on doing with him, but Presa are very good at working and enjoy many sports like IPO (bite work). Would definitely encourage you to get him into some sports!
Also for what its worth, having a vet that does have experience with your breed or breeds like it is important.
