It sounds more like your boss is being laid off than fired. Firings usually occur immediately and without time to pack-up/say goodbye. Many companies are finding reasons to lay people off, especially tenured employees, because the cost of having them on payroll is too great in this economy. It's so sad. My former company announced last year that 200 of us, 40% of the salesforce were being laid off - I was one of them.
Having gone through that recently, there are a lot of emotions that accompany the loss of a job. I suppressed mine until recently, and it was almost a year ago. I was so focused on finding a new job.
The hardest part is that all of your work "friends" slowly but surely begin to fall away as time goes by.
So, my recommendation is to do something sentimental initially. A group photo is nice, offering to take him to lunch would probably be my first choice (personally). But, most importantly, if he's important and wants to keep in contact, don't let the time be an excuse for not keeping in touch and helping him land back on his feet.
If he has been a great boss, it's probably because he wanted to see each of you develop personally and professionally and invested the time in you. Invest the time back in him in letting him know how much you appreciate it today, 3 months from now, a year from now, etc.