You can use Folin-Lowry's method for protein quantification

OR

Use RC DC protein assay kit by Biorad.

https://www.bio-rad.com/en-us/applications-technologies/sample-quantitation?ID=LUSP5JKAJ

you could concentrate the virus by something like this:

https://www.sigmaaldrich.com/technical-documents/articles/biology/amicon-ultra-centrifugal-filters.html

then measure the protein by Bradford assay.

The problem is the SDS. The Bio-Rad Bradford assay, for example, has a limit of 0.1% SDS (I think this refers to the concentration in the sample, not the final concentration after diluting with the Bradford dye solution, but I may be wrong about that)

(https://www.bio-rad.com/webroot/web/pdf/lsr/literature/Bulletin_6852.pdf),

whereas 1X Laemmli sample buffer contains 2% SDS. Therefore, you can measure the protein concentration by Bradford assay as long as you dilute the sample to lower the SDS concentration below 0.1% (i.e. 20-fold or more). Make sure to include the same volume of sample buffer in the standards.

Try it with the standards first to make sure it works, before using up part of your sample.

The IMHO easiest method is to spot the sample on a PVDF membrane (or, better, pass it over the membrane in a 96-well vacuum manifold), wash the membrane and then stain it with amido black. You can then either scan the membrane (office scanner, NIH-image) or punch out the protein spot and extract the dye with ethanol for photometric determination (doi:10.1016/S0003-2697(03)00255-0, doi:10.1006/abio.1994.1595).

If you are already performing SDSPAGE, another option would be to run protein standards, and quantitate by densitometry analysis.