It doesn't sound like an abnormal diet for a toddler to me! I think at this age there are probably more LOs who eat a limited diet of a few things than there are those who eat a huge variety, kwim?
I also would ask your doctor if they suspect something else going on - like, a 20 month old falling below a percentile is generally not a reason to refer them to any sort of special feeding or nutritional or dietary professional unless the doctor thinks that there is some underlying condition....even for babies it is usually not until they have dropped less than 2 centile units and for a longer span than a month...in a 20 month old you would not really expect them to grow and gain rapidly enough to get back UP a centile in a normal month anyway, they just really don't grow that fast at this age. So I would get some more information from them on what exactly they are concerned about....is it a huge weight loss and they are concerned there is some illness or is it just that he has fallen off of his centile iyswim?
Anyway as I said, personally, I don't think his diet is that big of a deal - loads of children this age hate veggies, and you can get most of the nutrients from vegetables in other places. Just offer them, put them on his plate, let him watch you eat them and don't make a big deal of it...if they are always there, he will eventually try them. And probably spit them out, but trying is a great start, kwim?
I would look at small ways you can make changes to improve the things he does eat, and that might help to open up his taste buds and keep you happier with his overall diet. So for instance he will only eat toast, make sure you are getting him a nutritious bread (be careful with TOO much whole grain at his age, it can be too much fibre for them to eat, so perhaps offering to a 50-50 bread for instance if it is currently white bread iyswim). Shreddies are good for soem good whole grains and they are fortified so he will get some small bits of things like iron and such..fish fingers, I would offer the best quality ones that you can afford, and maybe change up the brand of them now and again to get him used to change and different things and different tastes and not keep him stuck in a rut. If he is eating a few fruits, perhaps try to cut them all up and put in a bowl with a couple of new fruits, and say nothing, just give them to him. If he does not eat them, then eat them yourself and let him see you!
When my DS was in that 18-24 month stage I remember wondering how on earth he could survive on so little...seriously there were days where it was a handful of cheerios and some air
and I went digging for requirements at this age and it is seriously SO little...I will try to find some of the various links but I may not have them bookmarked anymore. I think it may have been something like 1000 calories per day and that adds up SO quickly. If you are offering him food, and not limiting in what he is taking in, then he will be taking in enough to sustain himself and grow. Adding in some good fats (like avocado, etc) are also some ways to add some extra weight on him.
But aside from that, it does sound really harsh to be making some kind of a nutritional referral just from a weight loss or centile drop unless they are suspecting there is some issue to be concerned about at this age....might be worth a follow up chat with your HV or GP, whoever told you that, and see why they are so concerned so that perhaps they can guide you a bit more as to what it is they think you should do and exactly how much weight he needs to gain...
