How can I treat this white stuff on my boxwood leaves?

I've had trouble trying to find the right diagnosis for your guy but initially suspect powdery mildew. If the issue begins with a brown circular spot under the leaves which then turn yellow and fall off it could be the dreaded boxwood blight. There is no cure but I'm not sure that's what it is.

Not knowing how you treated this already I'd suggest you thoroughly spray him down with Neem oil once every 10-14 days to see if it helps. Make sure to get the branches, stems and both top and bottom of the leaves.

I also suggest you cut a section off and place in a sealed bag (so you don't infect anything else) and take it to a nursery where they may able to give better information.

I agree. It looks like powdery mildew, but here's a link about a new threat to boxwoods:

https://www.pennlive.com/life/2013/05/new_threat_to_boxwoods.html

Yes it does sound like powdery mildew. Here is a good site that talks about this and even has several very good pics with plants with that:

https://www.thespruce.com/preventing-and-controlling-powdery-mildew-2539515

Thank you, thank you, thank you, all. I too think it is mildew. I am going to cut this all the way to the core and clean the remaining plant with soapy water and neem oil. I will do this every week for 4 weeks and get back.

Could it be powdery mildew? My lilacs get that. Powdery mildew is more of a nuisance then anything. . A few minutes of prevention will go a long way when it comes to this fungus. A few tips to take away from this are increase air circulation, check plants regularly, and when you notice White Powdery Mildew, take care of it. Heres a link with treatment suggestion.

https://www.gardenguides.com/778-keeping-ahead-powdery-mildew-garden-pest-tip.html

Call you county agricultural agent for advice. That's their job!

This may help if you wish to try it, I read this and it is interesting for me and my plants also.

Something to try on this situation or something else with plants?

Help Your Plants Grow Big And Strong

Hydrogen peroxide works by releasing oxygen. Because plants use oxygen to absorb nutrients from the soil, they love this extra oxygen molecule. This promotes healthier, more vigorous plant growth for your garden.

To encourage garden growth, mix one teaspoon of 3% hydrogen peroxide per one cup of water. After you pour the mixture into a spray bottle, mist your plants. If you want to cover an entire garden, increase your dosage to 35% hydrogen peroxide. You can also use this to pre-treat seeds. Since peroxide kills fungi, it also wards off potential diseases such as root infections.

Mildew/Mold use a copper sulfate, Bonide, treatment. It is even used by organic gardeners.

it is for sure mildew

Yes, powdery mildew.