What are some reinforcing consequences you can deliver either immediately following a practice session (or after you observe him giving a compliment or expressing thanks in vivo)– things that this particular student enjoys, wants, seeks out, etc..?
If you observe the student delivering a compliment to a peer, obviously you should wait to issue the feedback in private.
What reinforcing consequences can you arrange that are more naturally or intrinsically connected to this targeted behavior?
How comfortable is the student in receiving compliments? How does he feel after he is praised? Is he able to transfer the positive feeling he receives to understanding how the other person might feel after being complimented?