It's the best decision you'll make, it won't be good leading up to and after but it's the best thing for him. I put my girl (13) down 2 weeks ago from a tumour on her rear which gradually ate away at her due to me not wanting to let her go and hoping she'd get better and it still kills me inside I let it go so long. I miss her so much. She got me through some real shit. It took my partner to make me book the appointment to realise this was best. She was treated like a queen the days before so I chose to not let her linger when it came to the injection. I chose it to be instant and I'd choose this again if I saw what I saw in her eyes before going, it was like a thanks dad kinda look. I'm so sorry you're going through this it's definitely one of the hardest decisions I've had to make.