Your husband is who he is and clearly protecting, overly at that possibly is definitely his way of showing your son his love/care/protection for him. I don't believe he is doing this because he feels your son isn't capable of handeling important tasks as the tractor, etc. Yet, he loves him so much his protection for him just takes over.
This is why you are there Janet, you are the balance that your son needs. Your son isn't missing out on life's opportunity's because he has you there to guide him also. Your son is lucky to have both parents who love and care for him so.
Your son isn't missing out at all because of you being the "balance" over his Dad's over protectivness. He is truly blessed.
