R.R.
Hi there! A lot of people who will be attending will probably bring small gifts, or they will give him money. A very appropriate gift would be a tie, or you could get him a monogrammed white handkerchief or plain tie tack. He probably won't be wearing shirts that use cufflinks so don't bother with that. If he's going somewhere that will be cool, a nice scarf or pair of leather gloves. Any clothing or accessories should be conservative colors--navy, gray, dark red, etc. Ties can be more colorful but still conservative and tasteful and of course should match with a white shirt!
He will definitely appreciate gifts of money. Since he won't have room to pack anything that isn't useful, he will probably receive more cash than gifts. Most people will probably be giving him between $20-$50 dollars depending on how well they know him.
Since missionaries are not allowed to call home except for Mothers Day and Christmas (or family emergencies) a phone card won't be useful. They usually call their families collect or they will call from a member's home. He will still be in the mission training center on Mother's Day this year if he's leaving anytime soon, so he won't need it then.