But he's Fe Si Ne Ti
You're dumbing down typology if you think of it as just "Oh, they aren't rigid, so they can't be a J" or "Oh, they're an asshole, they can't be an F", etc. SFJs especially often don't "seem J". If you compare him to Jenny, tho, it's obvious she's the P and he's the J - he gets in a lot of bizarre situations because he has trouble saying no and wants to serve others and doesn't think things through (and this is where his lower IQ comes into play and makes it hard to type him - anybody can not think things through but with him it's definitely connected to his IQ and I see people type him all sorts of things), and more rarely just because of where his feelings take him which is where he can get really impulsive (ex. just running for a long time). Ultimately, though, his only goal is to have a stable life in his home in Alabama.
Jenny is totally opposite in thought, behavior, and functionally (and even says so herself) - Se Fi Te Ni
She purposely goes out of her way to experience things and keep options open as an END ALL while that's not Forrest's main goal (she travels all the time, takes up different occupations - being a coverfold, being a singer and musician, etc., does drugs and and joins different groups all for the experience itself and to explore who she is)
Look at another type Forrest is commonly typed as - ISFP; ISFP might have the P at the end but it's really a judging type, with Fi, a judging function, coming first. So even if people are totally wrong on what judging function he has first, they realize he IS a judging type (he can't be Fi because all of his values come from outside and he always adjusts to other people, whereas Jenny purposely seeks her own values and sticks to them - most likely love and freedom, seen when she joins the hippie and disco scenes; Forrest doesn't even think about what he REALLY values and just serves kindly helps other people in the moment - like an SFJ; he repeats the values of his mom and others instead of finding his own, etc.) When a judging type "judges" often they can seem like a P (this really only happens with Je, though, not Ji, being that it's extraverted and objective - not as stubborn as Ji).
He's also obviously Ti inferior
That's not all I gotta say but it's all I could piece together right now