Played in the Dutch second division last year at Go Ahead Eagles, this season (which has just finished, so should I say last?) he played for FC Twente and did really well there. Especially before the winter break, he scored some good goals and had a very decent season, while many were doubting at the start of the season if he was ready to be playing on the highest Dutch level.
This performance might get him a place in the Dutch national team squad. He's an energetic player, not really spectacular technically, but he can get past an opponent and has an eye for a goal, is fast and works hard. Jean-Paul Boëtius has way more raw talent, but he is younger and not as developed physically as Promes is, that's why I think Promes has a good change of making the squad.
But Van Gaal might decide not to include him in the end anyway. Holland already have Robben, Depay, Lens and possibly Kuyt as options for the wing who look like to be certain, and the 3-5-2 system LVG looks like playing doesn't really use wingers.
Nowhere near Manchester United level, nowhere near Ashley Youngs level yet even, and I'm willing to bet literally everything on it that he will not follow Van Gaal to United this summer.