How to encourage more fluids

Did you try some no sugar fruit smoothies or icecubes to suck on? Sorry I can't remember.

It's a bit difficult because if he is definitely not dehydrated or suffering in any way from it then I suppose that's good and there's no health concern, IYKWIM?
Have you tried different formulas?

Also, are you sure he is pulling off because of your milk (supply)?

Janini, my DD was like this over last winter but also refused milk and got quite dehydrated.

My suggestions would be...continue in a relaxed way to offer formula / milk and water from different cups / bottles.  When he takes a bit, praise him lots and hopefully he'll like that and catch on!  Have you tried a straw?  Some kids seem to like that because it's a bit more like a game.  Also, I guess you will be offering a range of food textures so if you need to, give him nice sloppy fluid filled stuff to start with (that way you know he's had some) and then finger foods.  Jelly, icecream, custard, fruit puree, thick vege soups are all good option.  Also whatever the organic brand of baby food is does a couple of fruit gels in jars.

If you are worried about dehydration I would check with your GP because the more reliable way to check it is capillary refill time (how quickly the blood refills empty capillaries) - they can show you how to do it properly.  Lots of the other signs don't necessarily point specifically to dehydration so can be misleading.

Good luck.  I didn't really find an answer.  When she was able to hold a bottle herself it stopped being an issue.

My DD is still a bit ho-hum about drinking much of anything most days. One thing I did do is take her out and let her choose a new drink bottle and a cup to drink out of. It worked for a little while till the novelty wore off. I have a range of drink bottles and cups for her now and I ask her to choose one each morning for her drink with her breakfast. She will usually drink a bit if she's been allowed to "control" the situation.

Also I don't really like giving her juice but I found if I put a splash of juice in some water it gives it a faint orange tinge and she thinks its juice so is more inclined to drink some.

Cordial is not recommended at this age. He really needs to be getting the milk into him. I would concentrate on re-building your breastmilk supply - it will only take 2-3 days.
Was there something, like teething or illness that triggered his decrease in feeding? Your supply WILL be low now since it works on supply and demand, ie- you produce to how much the baby is feedng, so cos he not feeding much your supply has decreased.
You can increase your supply but eating more protein, eating almonds, drinking more water, and regularly expressing- like evry 2-3hrs in the day. After 2-3 days of this your supply will be up and you may find he feeds better. Would be worth a try since hes not getting milk from any other source and breastmilk is the best option anyway! Good luck!