Personally I don't think you should go back to napping if it's not really what he does. Weekday routines may not be conducive to naps and naturally he may be tired and cranky in the evenings. But depending on interaction and choice of activities this may work to your advantage.
My kids would fall asleep in cars at very inconvenient times if I didn't engage them in a way that keeps them from caving in on the fatigue behavior. So I'd play games, save certain things they like doing for evenings, etc, so they are distracted enough so that when bedtime comes around 8PM they just fall right asleep. That was a tricky ecosystem to build because their inconvenient car naps and such would mean 8PM bedtime would require a parent to lay there for hours fighting the crazy - effectively making bed time somewhere closer to 10PM - and the horror of life is fully realized.
So we'd ask them to point out things as we drive, play board games and bath time, which they love, or play something like super mario brothers and this would keep them occupied enough to kind of gloss over how tired they were. Our youngest just turned 4 and she still caves in a little. Especially at dinner time, which tends to be around 6:30 or 7. That's the biggest time threat, which means playing word games with her or telling goofy stories at the table to keep her focused on anything besides being tired.
We were right there with you, and dinner is still challenging cause she will eat absolutely nothing if she's tired and that means she will stroll in around midnight claiming to be hungry... which basically spells doom for the rest of the night and tomorrow. So we know what you're feeling but I think phasing the naps out on weekdays was better for us and could work for you depending on what your evenings are like. Sometimes we have to work on things in the evenings so I get it if total engagement isn't an option, or is generally unrealistic.
Things our kids (4 and 5) like to do with us -
- board games like operation, matching games anything related to legos
- dollhouse stories, which means playing dolls however you feel fit
- bike riding outside
- jogging around the neighborhood
- playing video games, though the 4 year old doesn't quite get it yet
- pulling them around on a blanket
- couch forts
- dinner time stories (like the broccoli people adventures or
something equally insane)
They grow fast so it's probably just a phase that will be gone before you know it, but good luck. I know how hard everything is when you're exhausted from something simple like your kid sleeping off schedule.