Okay I am left handed and for most of my life lagged a little in areas like that. The reason it is very hard for a righty to show and lefty how to hold a pencil and vice versa. I remember struggling to tie my shoes and not succeeding and being embarassed, then I went to a camp and an older girl, who was a lefty, showed my one time and I've been tying my shoes ever since lol. My daughter is right handed, so when she was learning I made dash letters and let her trace those. I didn't try to hold/guide her hand b/c when I did she could not get it. I let her hold the pencil how she felt comfortable and it worked. When he colors etc he holds it in a natural( for him) way. Let him start there. He'll adapt as he gets older so don't worry. Oh, if you know any lefties let them show him. Good luck!
***I just read the other comments and someone said turn the paper to the left a little, you actually want to turn it to the right. I always do that. I never write with the paper straight.****