Attachment isn't fixed but is affected by previous relationships.
These days, attachment is usually conceptualised as organised or disorganised.
Most kids have an organised attachment style: this includes secure or insecure (anxious-ambivalent and anxious-avoidant). A minority have a disorganised attachment style.
If your son has a secure attachment, he'll have learned how to have his needs met and trust that he'll have them met. If he has an insecure attachment, he won't trust that his needs will be met, but will have a repeatable strategy for managing his attachment experiences. If he has a disorganised attachment, he won't feel safe either close to you or away from you, and this might lead to ‘fear without resolution’.
So, taking an example of him falling and grazing a knee (note: if he has sn, this might not apply):
Secure: he knows he can shout/cry/come to you/whatever; he trusts that you’ll effectively comfort him.
Insecure: perhaps he knows he can’t shout/cry (because this irritates you), but he knows he can come and sit beside you; he doesn’t trust that you’ll effectively comfort him (perhaps you’re distracted; emotionally dysregulated, or believe kids need to ‘cry it out) but he trusts that he’ll be safe in close proximity and takes comfort from your nearness.
Disorganised: bad things have happened as a result of his proximity seeking behaviours. Perhaps he’s been smacked, shouted at or shamed for making a fuss, or witnessed you become very upset/frightened as a result of his crying. He doesn’t know what to do. He can’t emotionally self-regulate, but he doesn’t trust you to do it for him and is afraid of what will happen if he approaches you. He isn't safe near or away. Perhaps he hides/pretends he wasn’t hurt; perhaps he screams and screams; either way, his stress is prolonged/unresolved.
So, while there's no checklist you can use, you can get a feel for his current attachment style by looking at what he does when he's upset/afraid. His fear itself won't tell you much, so I wouldn't concentrate on shyness etc.
Keep in mind that nothing about this is set. A securely attached toddler can become an insecurely attached child and vice-versa. A child with a secure attachment can have a traumatic experience, e.g. sexual abuse, and become disorganised. If you were unable to respond reliably or effectively as a result of your mental health in the past, that need not have any permanent effect at all. Today is what matters.