Tapping for calmness?
Loads of videos on You Tube
Simple, but teach him to take big deep breaths is he starts to feel worried, in for 5 and out for 5. It really helps!
I think lots of reassurance that if you need to come and get him you will, I think knowing he has a 'get out' clause will turn down the volume of his worry of being homesick and 'stuck there'
I'd also focus on the GOOD stuff what he will enjoy when he's there, getting his stuff ready, taking a teddy (they all do at that age) what new bits and bobs you can shop for and pack together eg: new toothbrush, facecloth etc.
Talk to school too, they would have dealt with this all before and will have ideas for you also. Perhaps a teacher he likes can talk to him and let him know that he can talk to him/her on the tripif he's feeling wobbly.
He will enjoy it, you know he will! The do so much stuff he will probably too tired to think about home sickness. You also need to not let your worry show, address his concerns but keep your worry on the down low.
I'd also not wave him off on the bus, too emotional I'd say. Do your hugs etc at home and take him to school as per normal.