Notice how he's sulking and is more quiet than usual. If his attitude seems to be a little different, even depressed, you can likely attribute that to him feeling sad about his recent relationship. Watch if he's moving slower or is easily distracted.
Consider his distance and ignoring you as an expression of his emotions. Check on him often if you notice that he's been locked up in his room for a while or is disinterested in doing his regularly scheduled activities.
Pay attention to abnormal behavior on the opposite end of the spectrum. When his attitude is oddly too jovial, he's probably fighting back his true emotions. Ask him how he's feeling about things, and see if he rants and raves about how good he is and repeatedly says, "I'm okay, it's okay, everything will be okay."
Note how he can't seem to stop talking about her. From the things she used to do to items she left behind, he's still in the zone. Consider that he's not quite adjusted to her absence and may be really thinking about her.
Be considerate when he needs a shoulder to cry on about his failed relationship. Allow him a chance to get his emotions off his chest by letting him talk it out. Realize that the sadness is likely to be overtaking him if he's poured out his heart to you.