He looks a little stupid/retarded but he isnt, i think.
He is very intelligent and smart, Even tough that he make is some short of dumb most times. He can understand the thing fine atleast
(I think run from the police because he was scared of get in prison seems like something that a retard will dont understand, Maybe) I read some things in the wiki of bully and by miself hearing characters quotes and for example here is a good one.
Donald : I heard Russell has an IQ of 7.
Just saying but i think he is not Stupid