German is not my native language but I have a lot of experience with it. To translate "because of what he said", I feel like I should say "wegen dem, was er gesagt hat" instead of "wegen dessen, was er gesagt hat". However, I'm unsure because "wegen" usually takes genitive. Both forms have a pretty large number of Google hits. What is right in this case?
In spoken language, I would definitively use the first approach
It sounds less formal. Using genetive constructions in spoken language may cause you to be perceived as overly correct in some regions (and depending a lot on your audience, of course).
For written texts, I would take the hint from Ingmar's comment to choose a less literal translation.