What Breed Is My Dog?

I adopted Buddha when he was 7 weeks old, and was told he is a Jack Russell mix. My friend adopted his "brother" the same time, but they look nothing alike. Buddha is 5 months old now and is pushing 20-25lbs.

He is super hyper and athletic, with long legs and what appears to be a docked tail. Any thoughts on possible mixes? The first picture is 7wks, second is 3.5mos, third is 5mos.

By Courtney H.