How to play first base

I think the other comments cover 1B accurately. The only time 1B might stradle the base is in a pick off throw for fly out where the runner is caught off base. A baseman (Second, Third, or SS) straddles the base in the first place to give the runner a single point in which to tag the base, which is the same point the baseman's mitt will be when he catches the throw and brings the mitt down to tag. If a runner is caught off 1st, the same principle would apply. The runner will not be coming in standing up; he'll be diving or sliding back, which is why the 1B straddles the base. But I could live with the other coach's comment and let him play the base as he will field it, i.e. foot opposite his mitt on the inside/infield corner of the bag, sweeping the tag back to the bag.

As far as playing time is concerned: The key to playing time is his bat. If he's hitting .600 with 100 RBI's, I can guarantee the coach will find a place to play him full time, regardless of how he fields the ball (although good fielding skills help make the line-up decision easier). Even if he isn't a slugger, but has a good on-base average (walks, etc.) and good speed and base running skills, he will play a lot.