Sure, you can make it larger, just use measurements. Here's a chart with actual body measurements for children - http://www.craftyarncouncil.com/childsize.html
- you probably want to do a size 4. Add 2" to the chest measurement, which will be the number of rows. The number of stitches CO at the body will determine the length, and you need the sleeve circumference for the cast on at the wrist. If you can measure something he wears that's not small for him, just use those measurements for sizing. Sleeves knit a little longer and rolled up, and length a bit longer mean he may be able to wear it for 2 years, so larger isn't always bad. Once they reach the toddler sizes, kids grow taller faster than wider.