He seems nice, but I think he's not all there mentally.
not all there
Frequency:
65
%
After the accident, he was not all there mentally for a while.
Idiomatic Expressions for not all there
Phrase: a few sandwiches short of a picnic
Meaning: mentally deficient or lacking intelligence
Usage: I don't think he's all there, he's a few sandwiches short of a picnic.