My advice would be to let it go tonight and tomorrow and then try and get him to wear it solidly through the weekend. I can guarantee that if he wears it consistently from Fri night to Mon morning, and talks loads at home with it in, by Monday morning it will feel much, much better.
For practicing speaking with it in, go with what your DS likes doing - for my DS it was unlimited time on the x-box as he always talked/commentated his way through games. Not online play - just on his own - but it meant he was in a room, by himself, and was talking/shouting loads. For DD - we encouraged her to sing all weekend as that’s always been pretty standard for her, so she had unlimited music time and she sang along all weekend.
Make sure he has plenty of tissues readily available in every room - he WILL dribble for those first few days.
Also have soft foods aplenty - even better if they’re treats. Lots of thick buttermilk pancakes with syrup and cream, soups, ice cream, cake, pasta with lots of sauce etc.
The first few days are definitely the worst but if you can get them over with, in the privacy of your own home, with no-one around to mock them for talking funny or dribbling, then by Monday there will be a big improvement.
2 DC had twin blocks and both were done with them 2-3 months earlier than predicted because they wore them consistently. Orthodontist said that the timing they predict takes into account that teens often aren’t anywhere near as compliant as they say they will be/say they are, so the fact my DC were totally compliant worked in their favours. In the long run it meant they also moved onto train tracks sooner and the whole thing was quicker than originally anticipated .