His sex drive should be up.
And NO, he will not have to wait 5 weeks for the Test-E to kick in. The Sustanon is a blend of tests, and has long-estered testosterone in it... therefore a high-test concentration is already built up in his blood-steam.
By switching to test-E now he should continue to see quality gains for the next 5 weeks.. (and waaaay less painful injections)