Go ahead and give him to build. My 6 years old build the truck by himself and about 70% of the excavator, the rest his mom build it. But for sure, he will loss his patience once in a while, just give him some encourage and it should be fine. By the way, my son spend a little bit over a month to build the whole thing.
P.S. He read the instruction better than my wife, he can point out her mistake. ?