just seems to lack any ambition of doing anything
That sounds like my son, OP. Certainly at school he did the minimum to pass exams; it drove me and his dad crazy because he just wanted to play computer games or do freerunning out of school hours. Aaarrggh!
This changed when his best pal moved away to another city some hours away to work, and he would go up to visit for weekends. He decided at age 17 that he would move in to the flat and live there too. He found a local college course and got a place. He then told us. We were surprised really because it was the first time he'd made a decision, a plan and carried it out. We did suggest he apply for our local college to do the very same course here. He did but it was full (so glad now, because had he lived at home I don't think he would have achieved so much in his life).
He moved up and spent a lot of time looking for work, any work. He had no experience so it was difficult. He had to though because he had to contribute to the rent of the flat (although pal let him off the first month). He got a chance of work at a 24 hour petrol station/shop. He worked a Saturday and Sunday and it meant getting up at 4am for a 6am start (he had to walk there, no buses at that time of the morning). He did it and was never late, did his college course Mon to Fri too; 100% attendance, passed everything. Got a 4 year apprenticeship when he'd finished, with the local council. Has plans to go abroad to work in the future with his trade.
He's a different person now to the "lack ambition to do anything". OP, your son needs a boost, something to work towards, to strive for. But you can't do it for him, it has to come from him. Maybe have a chat with him to find out what he sees in his future, what sparks his interest, a goal he can start planning for. Getting a job, any job at all, is a start. And there are jobs out there, not glamorous jobs albeit, unsocial hours that no one wants, dirty, physical, tiring perhaps.
My son said that he felt a huge boost of pride in himself when he started to earn his own money and pay his way. Sure, he hated the early weekend mornings and losing his weekends, but he saw beyond that, he saw that it was the way to get what he wanted; a flat share with friends. Two years later he has stopped the garage work because he can manage on the apprenticeship pay so he has weekends back 
.
I wish you every success, OP, your son is stuck in a rut and by chatting you can hopefully help him begin a plan of action.