Green Wig
What you’re looking for is a short, spiky wig to use. This is more of a blue-green hue than the actual green you want, but the reviews on this wig have been largely positive so it should be a good enough alternative. You may want to trim it down further (especially the sides)

White Short-Sleeved Polo Shirt
Wear a white polo shirt. You can also go for a button-down short-sleeved shirt though I think polo shirts are a tad more comfy

Overalls
Now for the fun (funny) part. He wears a pair of pants that are extremely high-waisted. Now, it would be difficult to get a pair of jeans with those kinds of measurements so I would recommend, instead, for you to wear a pair of bib overalls. Then, detach the straps, tuck it inside your overalls, then pin the overalls to your top to keep them from falling down. Just like Ferb’s, the pants of these overalls are already cuffed. If they come uncuffed, simply fold them.

Blue Military Belt
To further secure the overalls, use a blue military belt. The military belt is convenient because it is fully adjustable

Black / White Sneakers
Low cut sneakers complete your summer-ready outfit