Why wasn't Trevor Morgan credited as young Jimmy?

This might be a dumb question, but why wasn't Trevor credited for young Jimmy? I've tried googling it, but all I came across were a few different pages that stated the fact, but didn't explain the reason. I also looked up reasons for not crediting an actor/actress (since I was curious about some other people, too) and the reasons I found did not make sense in the cases I was thinking of. The reasons I saw were things like the SAG charging more money for more famous actors, so they sometimes go uncredited so they don't have to be paid the normal rate. Another reason was to surprise the audience by appearing onscreen unexpectedly. Also, trying not to upstage other actors was mentioned.

I can see how those would all be valid reasons, but I don't think they are really that applicable in this case. The closest one would maybe be the SAG one, except he wasn't really famous...maybe they just didn't want to pay him very much? I would think that parents/guardians would have made sure he got paid and credited for what he did...especially since he is the young version of the main character.

Anyway, do any of you movie gurus know? Please help me be less confused...