Firstly, don't worry excessively. If you have a genuine concern over the "lump" on his head then obviously you should act on that, but my first thought is that you are looking for a reason for, or a cause of his sudden stammer because, naturally, you want to fix it, but in looking for a cause you may have placed undue focus on the lump. Kids do bang their heads all the time, whilst playing, whilst at school or nursery, and usually, it is nothing to worry about. My daughter's school would place a sticker on them saying "I bumped my head today" so parents knew about it, but otherwise nothing.
There are many possible causes for the stammer:
- family history of stuttering
- family dynamics
- development during childhood
You may not want to hear this, but maybe your grandson is going to have a stammer, not due to anyone's fault, but because he was always genetically disposed to get one. If that is the case, then I'm sure you won't love him any less than if he was "perfect". Kids with stammers can get help from speech therapy which may mean he loses it, or learns to control it. But equally, this could just be something transient. Many young kids go through a stage between the ages of 2 and 5 when they stutter (stammer) and he is at that age. There are lots of other things like this that just go away by themselves. Depending on which study you read, either 1 in 10, or as many as 1 in 4 children will develop a tic at some point in their childhood, and most last no more than a year.
Keeping calm and not worrying the child by correcting him or pointing out his stammer will help. Pass on what you have observed to his parents, but don't try to solve the problem or suggest possible causes, as this may cause friction between you and his parents. Support them in any way they are advised to deal with it.