You can mix and match the pants with funky T- shirts and he would look the dude that he is. At this age buy clothes that are little over sized because this is the time of growth and if you buy the exact size the kid would outgrow it within a span of three weeks. There are travel pants, which also are comfortable for a kid since they are light and facilitate him to play, feel free and not uninhibited. Instead of buying the pants and the T-shirt separately, you can also go for the sets that are available in the market, in some of these pairs there are also matching socks and jacket.
