Gosh, you poor thing. You've worked so hard and have come so far.
Firstly, I would say that while your OH was not right to feed formula behind your back, he's clearly done so out of love and worry for you and your baby. The key is to re-direct that concern into support for the way that you know is the safest option, both now and in the long run.
Can you maybe give him information to read on the risks of formula feeding and the ways that breastmilk helps babies, especially those who have had a tough start? Here's some suggestions:
[[http://www.kellymom.com/nutrition/milk/infant-formula.html]
www.infactcanada.ca/RisksofFormulaFeeding.pdf
www.analyticalarmadillo.co.uk/2010/12/when-you-can-prove-formula-is-nearly-as.html
Both Kelly and the Armadillo are IBCLCs (highest level of qualification in lactation there is).
Your HV is kind of contradicting herself and of course she's wrong, which you clearly understand.
Tell me about your supply worries, why you are concerned, and also can you give more details about in what way he's not putting on weight quick enough? How are they defining this?
You might want to phone one of the helplines. They're open until 9 or 10pm and will really help you and also your partner if he will be involved in the phone call. All the people who answer the phones on the major phone lines are qualified "Breastfeeding Counsellors" which is around a 3 year diploma level training course specifically in infant feeding, rather than the HV who will have a few days training, unless she's done additional courses, which sounds unlikely given what she's said to you.
NCT: 0300 330 0700
ABM: 08444 122 949
LLL:0845 120 2918
BfN: 0300 100 0212