It is very typical for children from kindergarten to first grade to write certain letters backwards or upside down. Letters such as "b", "d", "p" and "q" can sometimes be confusing for certain children.
Talk to your child's teacher about how you can help your child best learn how to distinguish between these similar letters. For most children, they will learn to distinguish all the different letters on their own with time.
It is typical for children of that age to reverse letters to some degree. It's great that you are noticing specific behaviors like letter reversal, and I would continue to monitor his progress as he grows, expecting him to grow out of letter reversal over time. I would also share your concerns with his teacher. However, at this time, from the information you have provided, it doesn't seem like a concern that would require further attention such as a request for a psychological evaluation at school.
Backwards is very common. If he will practice, try "shadow writing" which is when you write the confused letter on the paper with a yellow highlighter. He can write over the "shadow" so when he writes, he writes it correctly. Try writing different words with the highlighted target letter in different places in the word (beginning, middle, end).
Also, take a dinner plate & pour salt in the middle. Have an adult trace the letter in the salt...erase & then the child tries. If you are not sure which letters, just take a piece of paper & ask your child to write random letters (not in abc order). If a child makes a letter backwards or upside down in your practicing, that is the letter you "shadow write" or trace in salt. You can also spread pudding on the table & practice writing in the pudding. Come back & rewrite the abcs from time to time & check on the progress of the letter you were practicing.