Based on what you said:
5 months, 27lbs
I think that dog will fill out to 40-50 lbs.
No bigger than 60lbs
Take a picture of him with his feet and his body, so we can see how big his feet are.
This "Jack russell"may have a cross.
If you think it's smart and well mannered you should hold on to it and train/teach it.....good dogs can be hard to find.
My friend has a plott hound and it's dumb as a box of rocks, always was and always will be.