My youngest son has been having intensive speech therapy from the age of two (he is now six). His speech was so poor to they wanted him to go to a special language unit rather than a main stream school. He was assessed and a programme was tailored for his specific needs. My biggest concern was that they were trying to make us embrace signing rather than learning to actually speak. My son is in the odd position of having poor speech but no other problems (such poor speech is usually accompanied by other difficulties). If ou are concerned about the process, contact these people -
http://www.parentpartnership.org.uk/
They are fabulous and will put your mind at rest.
I would also advocate asking for your grandson to be assessed for a statement of special educational needs (this takes ages!) but it will ensure the funds are in place for additional support when he starts school (I got my son assessed and he gets ten hours of support per week for his speech).