Destri from The Mother Huddle shares a free pattern and tutorial for making a little boy’s collared vest. The double lapel and faux welt pocket give it dressed up style, though these details could be omitted if you wanted to simplify the pattern. The pattern is in a size 4T-5, but I suspect that you could adjust it up or down using a shirt that fits your child as a sizing guide. Get the free pattern and tutorial.
[photo from The Mother Huddle]