This is a common complaint from cat owners! Many cats seem to not place high importance on drinking or they prefer shower/sink/pond water to their water bowl. Don’t worry too much about this, especially if Felix is eating wet food as this provides most of his fluid requirements. Also, if Felix has outdoor access he is most likely sneaking a drink from somewhere outside. In a 12 year old cat, signs of excessive intake of water may actually indicate the presence of diseases such as early kidney failure or diabetes so it sounds like Felix is actually doing very well for his 12 years!
With regard to milk, just remember that adult cats are lactose intolerant and should therefore only be allowed to drink lactose free pet milk. It is also preferable to keep this as a treat and not allow excessive consumption which can lead to diarrhoea, bloating and obesity.
A few tricks to try to entice Mr Felix to drink more water include the use of a water fountain as running water seems to tempt many discerning cats. Mineral water is also favoured by some felines over tap water (I have tried this with my cat and he loves it – he’d make an expensive dinner date!) Finally, cats hate stale water so be sure to clean out the bowl and refill daily.