#1 Get a Release of Liability signed by the parents
#2 Helmet (which you've already said you'll use, just putting on the list)
#3 I'd use Sunny but I'd lunge him for at least 30 mins and ride him for 30 more before putting the kid up. I'd also ride with the kid in front of me so I could hold on to him for a little while. Then I'd get down and lead for bit (so he feels like he can do it all by himself).
If, in the meantime, Nick gains enough weight and you can ride him enough to gain confidence in him and he is TRULY a beginner's horse, then you can switch at the last minute.