Me again (non expert!!). It may be that some of the 'issues' you are experiencing are control related in that he will eat more / different things when other people are around, but asks for yoghurt when you are there - this is perfectly normal and is his way of exerting his control in his family.
Again, if it was me (not the expert, remember.....!!), I would probably try and find more opportunities for other LOs to come over for tea, such as his cousin, so that he can see other children eating which encourages him to try different things.
I certainly think this is why our Katie eats all her food at nursery - she sees other tots having the same thing and wants to join in. She also eats more / different things when her cousin is around.
As I already said, I can't be bothered with fights over mealtimes, so I would rather present the other opportunities as often as I can than cause trauma (even if small!) in our family dynamic.
The things you are doing re putting a little bit of veg on his plate sound great to me - I wee nibble of carrot is great!
This is pretty much what I am trying to do as well, and it definitely seems to be working, albeit quite slowly - the other day, Katie had chicken, sweetcorn, beans and tomatoes (yes, a weird combo, but whatever works, right?) - things that she hasn't really eaten much of before, so I was well chuffed!
The advice that taygensmom has given you is really great - and, as she says, it all depends on your family philisophy as to what you want to try.
Maybe you should try deliberately / accidentally running out of yoghurt so that your LO, if he wants to eat, will have to have something else (this is perhaps a bit manipulative, but is something I tried with Katie when she went through a 'jelly / jello' phase - I kept showing her that the stock in the fridge was getting lower and lower until it ran out - after that, she ate something else - whew!! Can backfire a little, though, when they 'insist' that you go to the shop to buy more - I guess you would know if this type of thing would work for you).
Or, you could just have a yoghurt chucking out party - get your LO to help put all the yoghurts in the bin in a fun way to celebrate trying new things to eat - I saw this on TV once and, although I thought it was a little drastic, it really worked and within a few days, the little boy was eating loads of different things he had never tried before.
One other thing - you say he drinks through the day - is this water or juice or milk? I find that if Katie just has water (which she loves) and not juice or milk, she is more willing to tuck into her meals.
Oh, one more final thing (!) - I think that the variety your LO is eating is pretty good too - I think if you can come up with a little plan that YOU / your LO are comfortable with, then you will be able to achieve what you want to do.
Best wishes and hugs. xx