What do I do if my boyfriend keeps on insisting to visit her mother when he's out of the country? I already asked his mother if I could come over but she insisted that it's better if we talk via sms.

I would simply relay what she's saying back to your boyfriend. If your mother-in-law would prefer contact that way, it's not your fault and you shouldn't be forced to be pushy with her on his behalf. Just be open and tell him that you've tried to contact her and visit, but it's just not working out at the moment.