My initial impression for Forrest was ISFP. I favored ISFP over ISFJ because he seemed to have that SP sort of adventurous spirit. Can't rule out ISFJ though and alot of the explanations for him being ISFJ make total sense. So maybe ISFx?
5w6 or 9w1 sp/so/sx, I think
The Si part of Forrest's psyche is fairly obvious, his philosophy (such as it is) is completely dominated by memories of his mother's sage advice. But although more passive than not in his attitude, there is an Se side to Forrest which enjoys new experiences even if he doesn't seek them out. Is that his tertiary? But with Si versus Se, as far as what they feel inside, difference of intensity is the key to understanding Si and Se behavior. The Si seeks out intensity of experience, the Se experience is more expansive yet more superficial. I don't see Forrest as primarily either of these. For him, the Si serves to advise his primary function, it is not a way of life. This advice consists of his mother's wisdom. So the purpose Si serves depends upon its dominance in his psyche's heirarchy.
I think the Fe stands out for him more than any other function.
"I absorb energy like a sponge everywhere I go. It allows me to see the world and my purpose in it." Zak Bagans, Ghost Adventures (INFJ)