I also wanted to reassure you that I have heard a lot of good things about propanolol and hardly anything negative. My daughter was on oral steroids for almost a year for her hemangioma and it is my understanding that the steroids have more side effects than the propanolol does (this was 7 years ago when propanolol was not offered)...my understanding is that it is better than the steroids...that being said it is "newer" in the treatment of hemangiomas so time will tell, but I think that this drug has been around for quite a while for other uses and doesn't have a lot of side effects.
Judging from the size and growth of your son's hemangioma I would definitely go with the advice of the doctors and start treating as soon as you can. It sounds like they are on the right path. Keep us posted on how it works! I'm sure others will chime in that have used propanolol..I'm sure they will have some experiences to share.