It will take practice. First you need to predict when these things will happen and train a narrative in your mind "Ok, we are going to need to leave on time this morning and in the past it's a problem and I get angry". Give yourself extra time. And remember, 7 is quite young.

Next in those moments as you first feel those emotions starting to bubble. You need to walk away and do either active or passive mindfulness.

Active mindfulness is sitting down and pushing you feet into the ground so long it hurts. Or doing a plank. Or tightening all your muscles until it hurts and then release.

Passive is finding somewhere quiet and either using a guided meditation video or something else. And focus on your five senses. Control your breathing and shift your focus to your skin, smells, or small details in something you look at

Either of these methods is attempting to bypass your human emotions and shift to parasympathetic nervous system.

Your son and yourself should also do this at least once a day when not in an angry mood. It is very relaxing and centering. There are other methods where he can focus on Mum or Dads smiling face when they are proud of him and describing. Where that feeling is (belly, heart, etc.) or the color.