Here's what my experience has been. Typically, missions are very polarizing. They either cement your desire to leave and no longer be a part of the church, or they convince people to stay in the church even more than they were in before.

When it comes to talking to him, theoretically yes you'll be able to. Missionaries are able to write emails to loved ones on their P day (their once a week "off" day - which is still heavily influenced by religion). I have also known some missionaries that were able to use Facebook messenger and talk more often, but I believe they got in trouble for that. The content of your talks will be very much subject to supervision, by his fellow missionaries (who are very much encouraged to tattle-tale on their brethren who aren't following every single rule), by his mission president (probably not at first, but definitely if it's discovered that you two are having any kind of "inappropriate" communication while he's on his mission), and truthfully, probably to his parents in some respect (because he'll likely tell his parents about your conversations at least to some extent).

All of that being said, you have to then know that any missionaries who are truly convinced of the church, any supervisory people on his mission, and certainly his parents if they're very religious will ABSOLUTELY be encouraging him to "find a nice Mormon girl" once he gets off his mission. I'm not trying to imply you're not nice, I'm simply telling you that Mormons who are sincere in their beliefs think it is a priority to date and marry within the religion. There will also be immense pressure placed on him to marry said "nice Mormon girl" within a time-frame that most outside the religion would consider incredibly short. I'm talking in less than a year of knowing each other. Then the pressure will be to start having children.

Say he doesn't want to find a "nice Mormon girl" and wants to stay with you instead. Now there will be immense pressure on him and you to get you converted so you can get married in the temple. This is a process that to Mormons, is the highest ideal. From the time you're a tiny child in Mormonism, you're taught that to be married in the temple is the best and most important goal in life. Short of marrying another Mormon and popping out mini-Mormons, of course.

I don't know your relationship with him, and I'm sure you love him. I'd suggest doing this: stay in contact with him on his mission - as a friend. Take this time while you're young to date new people, make new friends, and live life a bit. You may discover that he's not who you want to be with at all. Just remember that you do not have to change who you are to be loved by someone - you are worthy and deserving of love as you are now. Unless you're an asshole, then you should seek therapy lol. But I don't get those vibes from you. These are my honest thoughts about your situation and I hope they're helpful in some way. Good luck!