Help choosing a shirt size for my boyfriend?

I want to get him this shirt but I don't know which size to get him. I don't want to ask him cause it be too obvious. I had bought the L size one but it was way too wide so I ended up returning it,tho i think the length was about right. He is average and 6"3 tall guy.

Here's the link: http://www.ebay.com/itm/270822373751?var=570033779...

If you scroll down to the shirt's image, on the left side where it says"regular/unisex t-shirts" ,below that there is some pictures on the sizes that zooms in if you place the cursor over it. Please give me your opinion!

idk if this is in the right category