In some places he is in the verge of being an adult (at 16 in Scotland), it is your job now to advise him not control him. If say he can't leave cubs/scouts of whatever without a "solid reason" I am not surprised he is not opening up to you.
ds(18) dropped activities around that age, I told him it was his decision, but we had a conversation, one of many about how important to his physical and mental health interests were, the types of struggles as a teen he will have, the decisions he will have to make and how he could always talk to us. He tried the drinking in the park, but as expected he liked the idea of being cool but it wasn't him as he hated all the drama around drunk teens and although we went occasionally he made the right decisions to not let his life revolve around it and find other friends.
Surely you want to have those type of conversations, advising and supporting him, rather than you holding on too tight and what is happening now?
ds(18) is now at uni has a healthy relationship with alcohol (for a teen), a steady relationship with a lovely girl, enjoys going to the gym/swimming/circuit training regularly for fitness, works PT, drives etc You need to talk to them where their choices will take them, then let them make them, they learn from their own successes and even more from their mistakes at this age. They learn nothing from what they are forced to do.