I don't know what the other moms have said, but I will check their responses later, I don't have much time at the moment.
Poor thing, you do sound frantic and with darn good reason. I am sure that your mom was livid when your son fell. She probably hurt and cried longer than your son did. She did the right thing, Ice as much as he would allow. Sometimes, contusions occur, these are bruising of the bone, but are not usually serious. Because he is so young, his bone structure is tender. The bump you are feeling may be from the bruising, (blood in the area, because the scalp and head are very vascular, (vein filled.) This will be okay. some bruises takes longer to heal, and on the head and face, for sure. Think of how long it takes a black eye to heal and equate it to that. It will be sore for sometime, maybe even after the swelling has disapated, and that is normal. You should not feel panicked or worry too much as long as he is not blacking out, having motor movement problems, or having redness on the whites of his eyes. (sclera). If he is not staring into blank space and drooling with involuntary movement of his extremites for periods of time, he is fine. It would still be okay to apply ice when he is sleeping for at least 10-15 minutes.
I think you are doing fine, just relax a little. Many of us mom's have been in this position before and we came out okay and so did our children. Give the bruising time, most people will tell you, everyone's bruising heals at it's own pace.
You are doing great, you are monitoring the site and checking for changes, you really cannot do more than that at this time. You are a good mom! If this is still a problem over 30 days from now, demand that your ped's check further. I think it will subside. this is from my nursing experience, but always seek a doctor's advice. Kiss the baby and hug yourself!