My child seems to have difficulties riding bike - would different tires help?
Not a helpful answer, but I would definitely find a lighter bike. In the UK, Frog and Isla are popular makes for light bikes. I would say a 20&quot; wheel is OK for a 5-6 year old.
A single-speed Frog 52 (20&quot; wheels, 10&quot; frame) weighs under 8kg.  Make sure the saddle is high enough, and raise it if he looks uncomfortable.
Both my children had Frog bikes and did fine. We bought ours second hand. They are very expensive, but maintain their value, and we sold both of ours for a profit when both children had outgrown them.
I doubt tyres will make any difference.