May I check, when lo turn 16 months old , he is still using Size L teat for his pigeon milk bottle, do I need to change to any other bottle or make the teat bigger ?

If he is comfortable drinking at that speed and flow, there isn't a need to change unless you are adding cereal into the milk. If you are adding cereal into the milk which makes it thicker in texture, you can consider changing into the Y teat.

I usually cut the heat to make it bigger for my boy. I don't folow the sizes I find that they don't match accurately especially for older kids