I have a 5.5 year old DD who was diagnosed in May this year. She has always been a really mellow kid with very few tantrums or meltdowns, particularly when younger. I would describe her as quite 'passive' in many ways and she generally just goes with the flow. She has just in the last 6 months discovered some defiance and difficulties with emotional regulation which has lead to some 'meltdowns'. In addition, she has become much more fixed in her desires and routines, which proves difficult at times. I think that although many kids on the spectrum experience meltdowns (for various reasons), there are also those kids who are more mellow and passive. It sounds like you're really on the ball with noticing difficulties he is having and implementing early intervention. The waiting and seeing is hard.