I personally WOULD feed the baby, only because my experience with joeys was that mom didn't have enough milk and they started to decline due to not having enough food... It's always okay to feed a joey as much as they will eat. You can get Esbilac Puppy Milk/Formula at the store, mix it with plain Pedialyte, and some apple juice and/or honey in it. Suck up the liquid with an eye dropper or syringe, and dab some liquid onto the baby's lips. Don't put it into their mouth, as they choke EASILY (and/or aspirate, meaning they inhale liquid and could die). So if they lick it up you dab it on their lips again, and however long they seem interested keep doing it. It won't take a lot.
Yes, hold the joey. Help get it used to hands, and to you. If it's small, hold it for just a few minutes at a time. Increase this as it gets older if you'd like. Mom can get really freaked out if you hold the baby for too long, so make sure you're within her view so she knows it is safe. The joey may cry for mom, it sounds a bit like crabbing but it's very distinctly crying. You can cup it in your hands and try to warm him up, try feeding him- or give him right back to mom. When you handle the joey, if it is small (meaning like the size of a thumb-ish) you MUST be careful to keep it warm. They can die within like 15 minutes of not cuddling up to something warm.
If you mean that you put liquid in a bowl, stop doing that. If the joey gets into it it could drown. Even if it's very shallow, not worth the risk. Give momma more protein- plain turkey, yogurt, baby food, mealworms, anything. Give her a lot so that she can have as much as she needs- her milk production is directly related to her protein intake, so baby needs her to have plenty of protein too. If the parents seem to be doing good, that's awesome. You shouldn't have to do anything, but make sure he doesn't get left alone ever, or have scratches on his face. Watch for other signs of rejection too. You do not have to feed the joey, I just personally would give it the option to supplement feed if it is hungry. If there's only one mom probably has plenty of milk, but you can't be sure!
GOOD LUCK! Joeys are amazing. =)