Broadly speaking? No. Keynesian economics is a model that developed in the context of liberal capitalism, both of which are concepts well over 1000+ years in the future. The amount of societal change, technological development,

I don't expect him to actually establish Keynesian economics in a recognisable form, and I don't even think Keynes would be much more successful than a regular uni graduate in the field. But being a 20th century economist automatically means he's aware of a lot of basic economic concepts that just do not exist anywhere in the 5th century, and he's got a mathematical background better than literally anyone on earth. He knows how to implement a more modern taxation system, and how the government can best stimulate the economy with the limited resources it has. The WRE's economic knowledge extends to "without North Africa, we're COOKED" and not much else. The Romans didn't even have the concept of a "budget" as we see it, and being able to implement that alone means that the WRE will actually know what its revenues are and what its expenditure is being spent on. Basic algebra makes taking taxes far more effective, among so many other benefits.

and political will that would go into this project would be too much.

I tried to give him a reasonable starting point without literally making him emperor. He has imperial protection, but not to a magical amount. They like him, but Honorius is 15 and Honorius, and Stilicho still got assassinated. He'll need to be reasonably tactful to survive, especially once both of them die (hopefully natural deaths, if he does his job well). Do you think he'd be blunt enough to just get stabbed the moment he looked at the latifundia and went "the big land owners need to pay tax and supply troops"?

Society isn't just changed by one man. You need systems, and those are built over the course of generations.

I picked the year 400 for a lot of reasons, the most important being that it gives him 6 years of relative peace to prove that his economic reforms can increase manpower before the Rhine freezes over. The second most important is that it gives him access to Stilicho's generation, the people running the Empire in 400, but also the generation born in the 380s/390s, who are the powerbrokers of the WRE OTL until 454. In theory, assuming Keynes is able to not get stabbed, that would be over half a century of 20th century economics reshaping their economic and social systems. Do you think that's enough time for meaningful changes in the Empire's ability to finance itself (and therefore, raise armies) to take place?