6-year-old suddenly very emotional and crying at school/activities – looking for advice

Looking for some guidance and thoughts from other parents.

My 6-year-old son has suddenly become very emotional and tearful over what seem like quite small things. He has two older sisters, and we never really experienced this with either of them at this age, so I'm struggling to understand what's going on and how best to deal with it.

A few examples:

During the summer holidays, I've taken him to a sports activity that he's been attending for around 7 months. He genuinely enjoys it and is familiar with everyone there, but on a couple of occasions he's suddenly come over to me during the session in tears. When I ask him what's wrong, he can't really explain it.

He's now gone back to school. He's been at the same school for 3 years, knows his classmates and teachers and has always been comfortable there, but we're seeing the same emotional reaction around going into school.

We recently had a routine doctor's appointment, nothing serious or frightening, and again he became very emotional and started crying.

For some additional context, he still sleeps with his mum. I've said for a while that I think we need to start moving away from this and encourage him to sleep independently. I've also noticed that when I take him to school, activities or appointments, he tends to settle quicker and cry less. When his mum takes him, the crying and emotional reaction seem to be stronger.

His mum may also be travelling abroad for a while in a few months, so I'm conscious that if there is some separation anxiety or attachment issue developing, I'd rather understand and work on it now rather than suddenly dealing with it when she goes away.

He genuinely comes from a very loving and secure home. He's well looked after, gets plenty of attention and is probably spoilt by the extended family if anything!

I will admit I'm starting to get frustrated by the crying, particularly because he often can't tell us what is actually wrong. At the same time, I don't want my frustration to make things worse or dismiss something that he genuinely doesn't know how to express at 6 years old.

Has anyone experienced something similar with a child around this age? Is this a normal developmental phase, could it be separation anxiety, or is there something else we should be looking at?

More importantly, what's the best way for us as parents to respond when it happens? I want to help him become more confident and independent without making him feel that he's doing something wrong simply because he's emotional.