Welcome to the site.
Well, it is quite evident that your partner is having some emotional issues that he is not able to address and acknowledge but he is using an escape from countering those unresolved issues and hence is avoiding contact with social life which includes you as it will bring to light what is troubling him which he somehow is choosing to ignore and he probably feels that is the best way to deal with it.
So at the moment if you try to persuade him by going after him to open up about his emotional roller coaster ride then unfortunately he will not perceive it as your empathy and genuine concern that you have towards him but he may probably see you as an imposing person, that is his persona I feel, so it will best for your relationship with him that you let him have his space and once in a while slip in a message or two about how is he doing and mentioning that you will be waiting for his reply and for his initiative towards resumption of meetings with you. Once you are able to get him to talk in person then you may coax him to open about what happened, what drove him away from you , what caused him to isolate himself, etc but certainly at the moment it is not the right time to look for answers or even to speculate about them based on the half baked information that you have from his sister.
I hope this helps.
Wish you all the best.
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