First part -yes, it is normal for a suppository not to work. I unfortunately have first-hand experience with this! (TMI, I know...) You can use another as long as the first one is completely dissolved and it's been a day, but if it's seriously impacted you may need to visit his pediatrician.
NOTHING will constipate like a lack of liquids! Not drinking enough water and/or juice will really do a number on him. Is there a reason he's not drinking enough? Does he just not want to drink? Keep going with the breast milk, but if you're giving him regular milk during the day -cut that out. Give him some prune juice -that will usually help. Also, while I usually don't advocate this, try some white grape juice. It is SO sweet that little ones usually gulp it down and it does have a laxative-type effect! In order to keep constipation at bay, yes, he needs to be well-hydrated around the clock with water and juice as well as his breast milk at night. Another "sure to poop" item we've found at our house are the little bottles of yogurt smoothies. DanActive makes one for kids and Activia claims to help you poop, but they all seem to work the same for my kids. My children also LOVE them. Again -they can have a lot of added sugar, so I usually limit their intake, but he'll probably want to drink several of them -so let him. It will usually REALLY get things moving!
This is probably not what you want to hear, but if he doesn't go -as I suggested, take him to the doctor. Sometimes they have to "dig it out" if it's truly and horribly compacted and hung so that an enema can't be properly administered. The doctor can also check to see if he has any little fissures or areas around his anus that may be causing him pain and therefore cause him to want to hold it in instead of going.