I think there may be some confusion over terminology being used by some posters.
In my mind, Mother & Toddler group is different to Playgroup. Playgroup and Pre-School are the same thing (known as Sessional Group in Ofsted speak). Nursery is different.
In a sessional group, it is not unusual for there to be children to be 2.5-4.5 years old. While ratio of the under 3's is different, the age groups can be mixed. Often a sessional group meets in a church hall / community centre, and thus there is only 1 main room, so mixing the children has to occur... and mixing the age group is actually encouraged as the older children can then mentor the younger children. A sessional group will be limited to a maximum of 26 children and run for a maximum of 4 hours per day. For further information, see National Standards: Sessional Daycare
lisalisa - I would suggest looking at local providers with a view to a gradual introduction from September. Meanwhile attending a Parent & Toddler group can help your son to mix with other similar aged children. Your MIL, or a nanny, could take your son to these groups.