The guides in the UK are indeed to ensure baby is getting 1 pint (500-600ml) milk per day. It may seem very low for some babies (meanwhile mine never managed that much). Whilst some babies will take, and need, more than this, it is the guide (based on WHO) so it is very normal for a HV to quote this figure.
Having said that the HV should take into account how much baby is taking, how big he is, and what his true needs might be rather than just fire off a number and expect you to just deal with it!
If there is a significant weight/health issue then I would plan a course of action with the guidance of a health professional, not a HV, a paediatric dietitian or nutritionist, ask your GP for a referral rather than tackling this alone. However most babies do not over eat so until you have further direction from a paediatrician I would continue as you are, feeding your baby to his need. Chances are he is not overweight and in need of significant cut of his milk but rather he is just big. What centile is he on for weight and height?
In my experience HVs comments can send mums into a panic that they are not doing the right thing, and then leave them to tackle the issue alone. I would at least get a second opinion from your GP (and if it was me, a paediatrician at the hospital) before deciding what to do next. It *may* be that a paediatrician or dietician suggests switching the 11 and 3 milks to a solids snack with a drink of water, he is very much on the young side for this so it's not something I'd recommend doing without professional guidance, but this route would quite easily reduce his milk intake to the 500-600 guidance without him 'going hungry'.
Things which would be quite easy for you to change without actively cutting his milk would be
- increase protein, at 6 months he can take meat and eggs, beans are also a good source of protein and may be more filling for him
- if there is no family history of nut allergy you can introduce ground nuts and nut butters (smooth peanut butter). Not whole nuts.
- ensure you offer water at meal times so he isn't taking milk through thirst alone but through hunger and the need for milk (if he isn't proficient with a sippy cup yet consider a straw or even a bottle of water to ensure he is able to get the amount of water he needs/wants)
- think about switching the baby rice for another food, baby rice may fill him initially but may not sustain him (for instance if he likes eggs a half or whole boiled egg or omelet would provide protein and good fats which will sustain him longer than baby rice, he may naturally decrease his milk intake once he is eating more proteins and fats)
Try not to worry, if you do need to reduce the milk I'm sure we can help you find a way so as not to turn your DS isn't a hungry grouch.
Let us know how you go with the GP/paed. They may just set your mind to rest and say to continue as you are.