He'll give you a comet shard after a few days you give him your Pokemon as a thank you.
This will not affect your Pokemon in any way. (wont lv up or anything). He'll just hold on onto your Pokemon.
SPOILER ALERT
After a few days, he wont give you the comet shard in person, you will just pick it up.
The Pokemon you gave him will be lying on the ground, there will be a thank you letter and it will basically say ahem he died and he ask you to take the Pokemon back and take the comet shard.