I used a mountain bike for my first tri (a very short sprint). I would suggest putting slicks on it - makes it a bit faster/easier on the road. No need for aerobars. They won't do anything by just slapping them onto a hybrid anyway. If he likes the race and gets himself a road bike, work with that for a while. Then, after a little more experience riding on a road bike, add the aerobars. Positions on a road bike versus a tri bike or road bike with aerobars are different. My suggestion is to get comfortable riding on a road bike first (no aerobars). This will help him learn bike handling skills, gearing, etc. Also, ride in a group (but NEVER use aerobars for this). It has a lot of benefits! Then, when ready for them, add the aerobars and only use them on solo rides and races.

But, for now, the hybrid with slicks will be fine.