The Father (not frum) of a Baal Teshuva asked his son for a loan. The son agreed to give his Father the loan on condition that he starts to keep Shabbos. Is this a problem of ribis?
Without getting involved whether telling him to keep shabbos is considered ribis or not, they problem may not even start.
The שו׳׳ע יו׳׳ד סי׳ קנ׳׳ט talks about lending money with ריבית to goyim and to non religious jews. It is מותר to lend money to a מומר (non-religious jew) with ריבית , one reason is because ריבית is only אסור when dealing with "אחיך" and they are not considered your אחיך anymore. The question becomes whether the father in this case is considered a מומר or not. Perhaps if he didn't grow up religious then he's considered a תינוק שנשבה who you would not be allowed to take ribis from. Although, if someone lives around frum jews and they still choose not to be religious, they're considered a מומר. This is not a psak... I personally would be afraid of ever stipulating a condition to a loan to anyone because ריבית קצוצה is very serious, and the sugya of mumar has a lot of different shitas.
The fact that keeping shabbos is something he's supposed to do anyways, may make it not considered ריבית.
Ribis is only אסור when it comes from the borrower and is payed to the lender (שו׳׳ע יו׳׳ד סי׳ ק׳׳ס). In this case the father keeping shabbos is not something thats "going to" the lender.
A similar question was asked by a Kiruv organization activist. May he promise someone a loan with the condition that he keep Shabbos?
I heard from my Rebbe Rav Pinchos Vind Shlita (author of Bris Pinchas and other ribbis related works) that if the activist has a vested interest or benefit in the borrower keeping Shabbos, then it would be forbidden for him to make this a condition to his giving the loan, since then it would be considered that the borrower benefited the lender in addition to his repayment of the loan and this would constitute ribbis.