If I understand correctly, receptions class is like a Kindergarten year.  I hope that is correct. I do not know what state you are in, but school programs in each state have some common goals, and yet can vary depending on city, district, etc.
My suggestion is to make an appointment with your son's teacher to discuss how he is doing and your concerns about his having too much work at home.  
See if the teacher can help you understand what is going on, and possibly help make the work at home easier for your child.

However, there is good news here.  You say he is happy and doing good.
This is the goal every parent wants for their children.  Things may not be as bad as you fear they are.  But do have that talk. Hopefully, it will help.

Bette J. Freedson, LICSW, LCSW, CGP

Hi!
At my school, in Kindergarten, homework was assigned every night, including weekends and some holidays.  I felt the same way at first, why so much here and they are playing games there, but I've seen the results and it is really worth it to stay involved and work with your child each night on the work that is sent home.

It reinforces what was introduced at school and your child will probably respond better to your instruction.  Just make it fun, at my school, we were allowed to be creative with how we did the assignments, for example, when they are working on sight words, Sight words will be an assignment for the night, but I could teach it any way I chose, such as through play or worksheets, whatever I felt best.  We, as parents, would write what activity we did with our child to return to the school.

I volunteer at the school, and I have seen the difference with the children, if a parent worked with the child each night as requested, those kids performed well.  In other cases, those children would spend time with the volunteer going over the assignments, which the child needed, but meanwhile they were missing the next subject introduction from the classroom teacher and falling even further behind.

Just have fun with the work that's sent home, I know that it's stressful and you're probably like I was and just don't always have the time, but make the time, they are only this young once, and before long they won't need you at all.  The homework itself it's not more than about 30 minutes worth + about 20 minutes of reading.  Enjoy this time and instead of focusing on what the school is not doing, focus on what you can do, because what is really important is your child.

Best wishes!