Did you get any literature on common reactions? I haven't heard of constipation either, but it doesn't sound like he's necessarily constipated anyway...
Personally I'd be more concerned about less feeding. Is he still having wet nappies? The less feeding could be because he has sore legs. When they kick their legs it's bound to hurt so I'd be pretty irritable too.
I'm not wanting to downplay your concern - if you are worried, take him back to the drs. But I have always expected some irritability within 24 hours of vaccinations. Generally though my girls have coped very well and have gotten by with extra cuddles and haven't needed analgesia or anything.
I hope he picks up and is back to his normal self soon.