My concern is my son is 8 year. Old and he is uncircumcised and his foreskin has never been able to pull back. His dr. Said it's a tight fit. I'm worrid?

Many options. It would be prudent not to try forcing the foreskin back for the present as that could trap the glans forward & set up an emergency situation. A consultation with a urologist can evaluate the issue & provide you a set of options.
Be gentle. It is normal for an uncircumcised boy to have "phimosis" trapping the head of the penis within the foreskin. Aggressively retracting the foreskin to force the glans out or tearing at the adhesions can cause significant pain and lingering problems including recurrent adhesions. If he is asymptomatic, don't worry. Consider Betamethasone cream if you have concerns. It will loosen >75% of phimosis.