I think it's great for your grandson. Perhaps a solid border in red or yellow color and then the flags as a second border would make you feel a break from the smaller cars.

He'll love it whatever you decide.
Andie

Personally, I think it is adorable. How about laying the parts you already have quilted on top of some of the colours that are in the four patch and see which one/s really make it pop and then use that/those colours for sashing? Better yet, ask your son which colours he likes the best to put around the edges. The wonderful thing about children is that they don't give a darn what is "supposed" to go together and just choose colours that they like. After years of teaching primary school I really get how wonderfully free children are when they choose colours to go together. Amazing thing is, it always seems to work and the kids like it all that much more because they had input into the decision making process. Your quilt is great. Have fun with it.