I've mostly done cakes for girls, so I'm at a loss. She wants a 9" round and will email me with the color details. I just don't want to make it look 'girly'. Any ideas on lettering, etc. would be great. TFYH!

If he likes sports that would help a good deal so just find out what so of activities he likes and go from there.
hth

If they don't give you any more information than that to go on try making the top of the cake a 16 in different OR bubble font?
Use any combination of blue, green, brown, tan for men/boys.

I made a hamburger cake for my 16 year old nephew, it was easy to decorate and a big hit with all the other kids. I also stole someone on CC site to use the white kit kats for the French fries.