I think Jentsje might be interested in becoming a full member (He is a Family member) so he were able to add his mother-side family tree and connect it if there were existing profiles of other ancestors.
If he's interested I wouldn't be able to help him in this process because of the language.
Jentsje has a lot of genealogical information to be added, e.g. there's a message he posted on 15 Nov 2014 on Liebbe Tjeerds Leistra's profile.
I don't know if it's difficult for him to work in WikiTree because of the language.