thanks guys for all your responses.
we also did some "research" on JobsDB and job positions with similar requirements usually pay 30-40k, sometimes even more.
so he will print out some of the job offers and he will show them to his boss. just for some support.
he wants to keep the job as he likes it and he doesnt have to work overtime and usually he doesnt have to work on saturdays.
so, i guess he will ask for around 30k.