I think the others answered well. Whilst true that Aspies are honest (to a fault) this is a generality. Since your son is 5 years old, he may "lie" like any other (NT) 5 year old. But, to add: I've learned that lots of Aspies, including yours truly (and I'm painfully honest) have imaginary friends, so I'm told.
Actually, another WP member asked this question a while back, posed to adult Aspies. I answered with a genuine "No." Then I read others posts. I have a robot named V8 (V8 is well known on the Wrong Planet, or so he likes to think) and to me V8 IS REAL. By this I mean, V8 is NOT imaginary. Imaginery means 'that which does not exist' and V8 is certainly tangible. Indeed, I made V8.
Not too long ago, one I know at University (NT) just casually mentioned in passing whilst in the hallway, "That plant isn't real." No. That plant IS REAL in that it's NOT imaginary. I can touch the plant and she and I both SEE that plant. I think what she meant is that plant is plastic, not alive. That was tricky for me to understand. Similarly, my teddy bear, Gottfried, is not my imaginary friend. Gottfried is definitely my friend and he is real (i.e., not imaginary). Gottfried has a distinct personality unlike my own and Gottfried is Neurotypical.
Does that help? Albeit, your son is 5 years old and likely doesn't mean to lie with ill intent. But, no worries. When he's more mature he'll be just as painfully honest as most any Aspie
The notorious but all too true story: NT mother to her adult Aspie son "Does this dress make me look fat?" Her loving adult Aspie son, upon deep contemplation, "Yes." If you want an honest answer ask an Aspie. But maybe not when they're 5 years old!
Welcome to the Wrong Planet, writer9. And to your son - WP has a Kids Crater Forum he may like.