I definitely agree that children can learn from observing and copying those older than themselves and likewise benefit from the opportunity to care for those younger than themselves. However, bear in mind that it is unlikely that all the credit for your son learning his alphabet can entirely rest with those older children - the early years practitioners were probably incorporating it into the activities across the setting (songs, games, resources and displays) even if they were not explicitly teaching it to him.
Also, mixed age-group provision tends to work quite well anyway in the 3 - 5 age range -all 0 - 5 year olds fall under the Early Years Foundation Stage Curriculum whether they are in a nursery, pre-school, school nursery or reception class. A five year old and a three year old will both benefit from play based learning and an environment with plenty of exciting practical activities. This curriculum continues until the end of Reception and is then phased out (often quite gradually) in Year 1 when the National Curriculum becomes the statutory requirement. Some mainstream schools have mixed Nursery/Reception units, so it is definitely not unheard of for younger children to work in this way.
However, as children get older the differences in attainment become much more marked - the lowest in one year group and the highest in the next year group are often poles apart! Teaching mixed age-ranges is generally regarded as very tricky and requiring a lot of carefully differentiated tasks.
I took careful note of the particular piece of work that I saw when I went around, as it was one of the last things I taught to my class before I went on maternity leave. However, say I had been teaching it in Y3, the class I saw at the Montessori school was Y3/4, but the level of challenge being offered was lower than that I had presented to my more able Year 3s....