Have you tried a different approach to time out? It did not work for me with DD, I think she sort of liked the drama of it all. I also had mixed feelings about it as no one wants to see their child distressed/screaming and I felt I was breaking her spirit. Also I believed she really did not understand what she was doing - it wasn't a conscious choice to be naughty. You need to find a system that chimes with your heart and stick to it - a few weeks is not enough. Things can take a month or two to start showing improvement and progress is not linear.
I found reading The Explosive Child helpful (this was a good few years ago and there are also more recent books like The Strong Willed Child and How to speak so kids will listen etc). Some things that worked for us:
Changing routine where it causes a flashpoint and sticking to a revised routine/structure - e.g. getting dressed before breakfast as changing after breakfast was a flashpoint. laying out the clothes the night before.
"Don't sweat the small stuff" - pick your battles. Does it really matter if she won't wear wellies on a wet day but insists on plimsolls - she will get wet feet - so what? Bring the wellies in a bag. Avoid the meltdown.
Giving the illusion of choice - e.g.instead of saying "get changed" - say "which top would you rather wear the red or the blue?"
Instead of reacting to bad behaviour - super praise good behaviour.
When they are quiet/playing nicely instead of thinking "thank god" and having a sit down - engage and ask questions - what are you building, that looks great , can you show me it when you are finished? and praise him. Attention is given for nice play rather than for bad. Show it to dad when he comes home - so a fuss is made about a good thing.
You know your son's triggers - are there any you can avoid? My DD used to go into meltdown in the supermarket- so I had shopping delivered and DH picked up extras like milk on his way home from work. i had 3 other DC so there were things my DD hated such as school pickups and drop offs for the others which we couldn't avoid but it lessened the load.
Good luck