Maybe your son has already told you this but just in case: The general rule of thumb is that serif fonts are more readable for print and sans-serif are more readable, especially at smaller sizes, on screens. Serifs are the curly, knobbly bits at the ends of the lines that make up each letter. You generally don't see many serif fonts on the web.
Another rule of thumb is to use as few fonts as possible. In most cases, one should be enough. If you use too many, you end up with a garage sale/car boot sale poster effect, which undermines the impression/impact of your writing... unless, of course, you're writing about garage sales/car boot sales.
My personal favourite is Trebuchet MS which is very similar to Arial, Verdana, etc. except that it has some neat details like a capital "I" looks different to a small "l", making it less ambiguous to read.
I've also been experimenting with Ubuntu, an open source font. I like the look of it but unfortunately, Ubuntu isn't installed as standard on PCs so this involves some complicated shennanegans to get it to work on everyone's computer. I stick with the most widely used fonts to avoid complications.
I hope this helps!