If you have eliminated all probable answers, then what remains, no matter how improbable, must be the answer... (paraphrased)
Do I honestly think your child is possessed, no - but I have played Ouija before and well, yeah- less said the better.
Did he happen to have a head cold? That could have distorted his speech? I've taken some Spanish in University and I only recognized the Madre (Maybe he was saying mother in Spanish and picked it off of Dore or other similar programming) - however all the romance languages have their roots in Latin.. Give Matt some paper and crayons and ask him to have his friend to write out why he's mad- if he's spelling entire sentences in a foreign language, then I would be very concerned. (if it's just a made up language you probably will just see a jumble and it's just a kid trying to make sense of his world the best he can.)
Ask him to draw his favorite cross. Ask about general info about his friend - hair, eyes, skin, age for examples.
As for the basement - I used to sleep walk when I was young (very common for young kids) and to this day if you ask me a question while I'm asleep I will answer and have entire conversations I won't remember when I wake up. Combine that with him mumbling sleepily back to you and then waking up - I can see this easily happening.
Don't try to make sense where he was: sleepwalkers don't need lights, as for the location - I ended up in random places like stairs. It didn't always make sense.
I think you were anxious about Ouija and then seeing these less than every day occurrences has given them more importance than they warrant. I do not mean this meanly, just that we have ALL psyched ourselves out before.
As for church - make sure this is not him acting out or that he doesn't have a justified reason not to go, Sunday school bullies in his class, undiagnosed A.D.H.D. that makes it especially hard for him to sit still and silent that long, eye problems that cause eye strains and headaches, an adult he feels uncomfortable around that he ONLY sees at church that makes him associate church with a place he doesn't want to be.
If you've eliminated every other issue(s) and you still think he has a demonic visitor - speak to your pastor. I would keep a journal, write down as much about each episode as soon as possible. If you go in hysterical, you might have people labeling you as an unfit mother.
It's a lot harder to think someone is lying, delusional, etc... when they have 'proof'. Plus a journal might show you a connection you have not thought of before.
I hope this helps, and best wishes sweetie.