No real advice really, other than my DS was very picky and I had huge struggles trying to get him to drink his formula and then weaning him on to solids. I was still struggling when I went back to work when he was about 6 months. My fab nanny got left to sort it out.
He certainly drank more when we switched to cows milk, I think he just didn't like formula milk.
Now he is 21 months and still does not appear that fussed by food - he can go for long periods of time on what seems to me very little and then he will just decide to eat. If I want him to eat then I have to distract him with stories, songs etc...while i attempt to shovel food into his mouth. The other thing that he does is graze, I just give him lots of opportunities to snack.
On the plus side, food does not rule his life, he isn't that keen on ice cream, biscuits or chocolate and tends to eat really healthy things like raw vegetables ( including raw onion !!).
That is all in the future for you, but I would just keep a close eye on him - get him weighed regularly and make sure that he is not falling behind on his weight centile. If he does then discuss with the HV how to deal with it.
We also gave our DS liquid vitamins - not quite sure when you can start that but check with pharmacist.
In relation to fluids, DS couldn't get the hang of the non spill beakers - so we use one where it ends up going all over him, but at least some goes in his mouth. We use one from Sainsburys - don't know the name but it is opaque with a design on it and has a spout with three holes.
Lastly, it totally stressed me out making feeding a battle - retrospectively the more I struggled to make him eat the more he fought me. At the end of the day he will take in what he wants.