Given his color and lack of ped I wouldn't call him a pit bull. Best guess would be a bull breed mix.
At that age without the ped the only way to guesstimate size would be to take his weight at 4 months and multiply by 2. It'll give you a rough estimate of what his weight would be.
He's wicked cute and reminds me of my boy.
"i can explain it to you, but i can't understand it for you."