I am very much sure the transition from Montessori to a regular school won't effect him, as he's just 2.5 years old and still have 2 to 3 years to go. By that time he'll be bored of this Montessori and will be ready, mentally, for a regular school, but with
lots of love and a little bit of persuasion, coz nobody likes to venture out from their comfort zone as well we adults too!!!
Same here as well. Another 1 and half year, my kid will be 6 and off to a regular school and I believe it'll be fine with him as well your kid.
And about getting admission to
grade 1st by the year 5/6 won't be a issue in terms of schooling, as it's the right age for grade 1st.
Hope this ease you a bit. Need not worry.