The Prima Official Game Guide states: "Should Laila ever become exiled due to the Empire gaining control of Riften, Saerlund will remain behind and live in the Keep in defiance of his mother's beliefs".
It mentions this on the UESP
https://en.uesp.net/wiki/Skyrim:Saerlund and I think it's correct in this case. It makes more sense for him to stay and it is what was probably intended by the devs. USSEP makes it so he is exiled too, which goes against the reasoning above.