If he has EVER held a passport he must declare it lost and apply for a replacement. They will check and there will be problems if you just say it is a first passport. My son was in the same position he lost his passport years ago. The problem is someone could have your sons passport and use it to get another one. A passport is technically the property of the government , you are just the 'holder' you don't actually own it.
They will trace his old passport from his name, date and place of birth and details of his parents, all that is on record. They can still trace it even if his parents had never held a passport.
Not sure but I think you can apply for an adult passport from age 17 years and 6 months if you are going to use it to travel after your 18th birthday. The passport will be dated to start on his birthday.
Children nearly or over 16 years old
Your child must apply for a 10-year adult passport if they’ll be 16 within the next few weeks or they’re already over 16. They can apply without getting parental consent.
I lost mine so reapplied; but I had to report the loss to the police and produce the police form with my application. I don't know if this still applies.
I found it months later, but didn't bother telling anyone.