It sounds to me it is the opposite, the line is too light, same type of problems I was having with my old cheap kmart special, it just wouldn't load no matter how much line I had out.
Look up the "common cents" sticky and use the penny test to measure the rods flex.
My rod is marked 6wt, but when I put it to the test, it came out a 9wt, I bought some 9wt line and what do you know, even with my su..su.. stinky casting ability, it worked it self out, so now I have a nice 9wt 8.5' should I feel like bothering some bass this summer.
Good luck, don't sweat the cheap rod, it's better than no rod
Stick a nice Martin 65 on it and go have fun with your son.