哈哈，that's all U need to say to him:

"I'd love to help U, but I'm so sorry that I'm so busy w/ such & such ..", We just need to say no in a nice way...

that is, unless you may need him to return you a favor down the road.

normally I love helping people in my work place as long as he/she is not abusive...
Helping people does not hurt us anything...