Does that mean he's been served carrots and potatoes 3 days in a row? Even 2 days in a row I would find off putting, certainly not appetising.
When you say he'll only eat beans, pasta, banana is that REALLY true?
"He eats loads at breakfast and lunch, but barely anything for tea" that's normal and is in fact how we all should eat. Most cals at breakfast, bit less at lunch, light meal for dinner. Are you serving too large a portion at dinner time? (Lots of people have lost sight of portion sizes - I made this mistake myself).
Toddlers are also winding down for bedtime in terms of biorhythms too.
www.nutrition.org.uk/attachments/article/734/BNF%20Toddler%20Eatwell%20Leaflet_OL.pdf
That lunch sounds VERY fruit heavy.
"He doesn’t speak yet so he can’t explain what he does and doesn’t like." He's expressing this by what he does and doesn't eat. Even adults want different things on different days.
I have (now big lanky streak of 17 yo) dd who still isn't keen on potatoes, HATES chips and chocolate. But eats plenty and is healthy.
If he'll eat the pasta with sauce you can sneak stuff in the sauce, with beans - easy to add onto pretty much any other meal or as beans on toast will he eat with ham or chicken slices on the toast and then beans, marmite, cheese either on the toast or grated over the beans? Does he like egg?
"so I have to send a pack up." Replace some of that fruit with veggies (cucumber, cherry toms, pepper cut in strips, even broccoli and Cauli can be eaten raw if he has enough teeth and is strong enough to crunch it - and of course carrot sticks), I'd also add a yogurt/from frais to that lunch, or was the cheese instead of that? what was in the sandwich?
Beans are actually a pretty good food (fibre, calcium, protein, potassium, magnesium & b6), if you use egg pasta that's also good (protein, iron, magnesium) and as he eats well the rest of the time I wouldn't worry re nutrition.
True what pp said about keep putting a little of other things on his plate and he will eventually eat them too. Also let him "steal" things from your plate.
They can be little buggers in eating better for others than mum too (I've been both sides of this as 1st a child carer then a mother - I swear I must be the only mum in the Western Hemisphere with a child that wouldn't eat smileys though!)
"Breakfast today was a bowl of shreddies, a slice of toast, a banana. Then an hour later at the childminders a bowl of porridge." That's a LOT for 18 months!
What are the other mindees eating? Is childminder too quick to give a sweet alternative to veg/meat/eggs?
Will he eat ravioli or other filled pasta?
Re egg - you can get fun gadgets that can be used to make boiled eggs into fun shapes (my dd hated eggs at this age but likes them now). I think at the time I got some similar from Lakeland.
www.amazon.co.uk/gp/aw/s/ref=nb_sb_ss_i_1_13?sprefix=boiled+egg+sh&crid=3E0U10Z9CXUX9&k=boiled+egg+shape+maker&tag=mumsnetforum-21
There's also things to make veg fun
www.amazon.co.uk/gp/aw/s/ref=nb_sb_ss_i_2_5?sprefix=veg+c&crid=1RQQ5NCWBX2SA&k=veg+cutter&tag=mumsnetforum-21
With one of my mindees who "didn't like veg" I went all 70's and did tomato zig zag cut etc worked a treat. I also used food dyes so he got to eat 'blue for a boy' mashed potato (yes yes I know gender blah bla bla but it got him eating veg).
If you're a baker (or granny is) - carrot cake, beetroot choc cake to go in his pack up?
Also depending where you are could the hot/humid weather be affecting him? Dd and I were chatting as I do my order on a wed, we're sick of salads but not wanting hot food so were thinking what to have instead.