I think you’re sad about him growing up OP, and I agree the limited range of colours for typical boy clothes is a bit depressing. It is sad to watch your baby have to conform with school age norms.
But there won’t be a single other boy at school wearing anything gingham, so you really can’t buy him those shorts unless you want to make him feel really really awkward about his clothes at school, and why would you do that. Just get him some nice colourful stuff to wear at the weekends to cheer yourself up. He won’t care either way, so don’t make your feelings about this into his problem.