Normally it would be: YO-khəm , just like
Jochem. The G and CH are pronounced the same in
Dutch (making having both of them right after one another a bit redundant). The U and E are both ə in an unstressed syllable.
Only Frisians tend to pronounce the G like the way the English do, like in
Abigail for example. Making it possible it's said as YOG-əm, if you're very sure that this person lived in Frisia, everywhere else in the Netherlands it would be the same as
Jochem.