I think what you're describing is pretty typical kid behavior. It's a great sign that he's doing this because a lot of ASD kids have trouble with this kind of representational thinking. If I said to my son, "Hey, that half note looks like a lollipop!" He would say "No. That's a half note."
That's great for your son!

Can your son look at books of insects and see the similarities between beetles vs. butterflies or other insects? (If not, have you tried the gluten-free, casein-free diet for a month or 2 weeks very strictly...1/2 a crumb might negatively affect the trial for two weeks -- see the book Healing and Preventing Autism by Jenny McCarthy and a pediatrician.)

He (and maybe most boys especially) like anything that moves I think. Between age 8-10 he has studied all kinds of planes now, has checked out books on tanks, weapons (I am surprised his school library has such books!), inventions.

Now we have a zoo/aquarium membership. I wonder if your son would see some interesting similar shapes in this sort of place!

Well it IS a cognitive skill to look for patterns and see similarities. With my son, we had a train track and one train and even now he likes and studies all kinds of trains. Somehow he started collecting little cars but then when I noticed him crashing them together, I encouraged him to check out insects.

They were outside and readily available in the summer. He was able to categorize, notice all the differences and similarities - we have both learned so much. I think he made amazing discoveries - like once noticing a black widow type spider (maybe saving our lives!) - and noticing that ants even move pollen around from one plant to another!