I have the answer.
Just chill and relax.
Porridge is super nutritious. Oats is a super food!
When my DS was that age he only at scrambled egg, toast, banana and milk. And pasta.
DH and I turned it in a huge pressure thing until I said I did not want to live like this.
We pretended not to mind, we had dinner together where possible, offered him his "safe" food and often put a little bit of something new alongside.
No pressure, no threats, no breathing down his neck.
And over the years he started to eat better.
We just pretended we did not mind, and pretended to be calm, until being really calm and casual bout whatever he ate (or did not eat) became second nature. We did not even coax him to try new stuff. We ignored the issue until it went away.
He became more adventurous over time, and now eats completely normal, anything we eat.
You cannot make a child eat, and turning dinner time into a battle field is the most counterproductive thing you can do, in my very limited experience!
As long as you don't stuff him with junk food (which you don't), eating the same old safe food does no harm. He'll branch out bit by bit.