What do you mean by enough pressure? Do you mean that the BB’s are rolling out the barrel? Or that there are not getting out the barrel? Or that they are just not getting the speeds you want?
I am not trying make you sound like a fool just trying to understand the situation.
I have an old style as well from 1978, and I am getting about 450 fps from it, but I have never cleaned it.
I have done no repairs or replacement to it tho, so some parts may be worn out too.
I hope you get proper velocities and that your son likes the air rifle.
Thanks
880 Fan