My sons 4 front teeth were pulled out when he was 18months old. He is now 7. His adult front teeth have not grown in yet. Is there a problem with that or is it normal?

This could be normal but adult teeth usually come in between ages 6-8. Read this for more info How Long Does It Take for Adult Teeth to Grow In? - Dental Health - Everyday Health

Since the teeth were removed at such a young age there is a chance that they have drifted. You may want to have your dentist take an x-ray of this area. Pull your sons upper lip up and take a close look at the gums, you may see that they are slightly blanched (white) or if you run your finger over the gums behind his lip you may feel a couple of bumps, if you see or feel this you are probably feeling the teeth and it's just taking a little longer for the adult teeth to erupt since they didn't have the baby teeth to guide them into position.