Hi there, I am a speech and language therapist and to be honest at your sons age most of the work needs to be done through parents- ie SLT giving parents advice on strategies to be implemented in the home/nursery environment. At 2.5 it is very difficult to engage a child in any sort of 1:1 therapy as they don’t usually have the required attention and listening skills. Children absolutely do develop at different times and in different ways, milestones can be really useful measuring tools but also great sources of stress for parents so looking at the overall picture of how a child is developing is often most useful. For that reason an initial SLT assessment of a 2.5 year old is likely to be very play based, looking at all of their skills and interaction and involving parents a lot to gain a picture of what the child is like in various environments. There are loads of lovely resources online about how to help create a language rich environment at home which you could try in the interim while you wait for your appointment, this is a good place to start.
www.thecommunicationtrust.org.uk
Services are different depending on where you live, but they are generally really really stretched which is so rough for families and therapists alike. Good luck with it all!