If he only "just" went to Germany, it might take a while for the Border Force's entry information to be sent to his PPU ...
There is an RSO (more than downloading, served several years in prison) who went to the US on holiday without notifying. He got into the US no problem by ticking "no" on an ESTA, but was arrested on return once the PPU got the information about his arrival from UK Border Force. He was sent back to prison. The story can be found online but I'm not sure I should link it as we all feel the same way about privacy and the Google effect.
As an aside, this would suggest that unless you tell the US about your conviction, you're very unlikely to have a problem when travelling on an ESTA, but it would be best to wait until you no longer to notify under the SOR.