If you absolutely don't want to use 3rd party software(including open-source), then just simply paste in your wallet address or qr code. Or if you want to be slightly more fancy, add a donate button(on your menu bar or at the bottom of the page) and use a modal pop-up with your wallets address/qrcode when clicked.
This is good suggestion and he can simply make changes in CSS to give it a more handy look that doesn't display the address directly and it's hidden and on the click of button it appears on the screen of user in full clarity to avoid any confusions.Even if you don't know about it there are tons of tutorial available from where you can learn how to do it.