ReisomNomad gave you some good pointers but my workflow would be a little different, and I think simpler to do. Here it is in a nut shell. You generally want your compositing to follow what would happen in the real world. You want the lion between your actor and the background so that is what you need to do. Until you actor, the boy, moves in front of the lion you can't tell where he is because you are are creating an image on a flat screen. Because you have introduced 3D camera tracking and have made the lion a 3D layer, and done a fairly good job of blending the lion into the scene the biggest challenge is when the actor is blocked by the lion. Think about what the shot would be like if the lion was really in the scene. The actor would be in front of the lion so that's what you want to do in this case. To get the actor in front of the lion you need to remove him from the background. Let me go through the exact steps I would take.
- Move down the timeline until you are 1 frame before the actor goes in front of the lion
- Duplicate your footage layer and set an in point for the top copy
- Move the top copy above the lion layer
- Double click on the top copy of your footage and use the Rotobrush tool to mask out the actor. You only need to create a mask where the boy passes in front of the lion.
- When the roto is complete refine the edges and do what ever is necessary to get the actor to blend in with the lion so it looks real
The advantage of doing this is first, you have now arranged the foreground(actor) middle ground (lion) and background of your image in the same way they would be in real life. This makes compositing, adding light wrap, blending or hiding the edges of the foreground and basically everything else easier. Second, you have a perfect visual reference to use to create and refine your matte. Third, you are not wasting any effort masking parts of the shot that are not necessary to complete the effect.
Always think in layers. Always try and imitate real life. Don't spend any time doing hand work on things that you don't have to. I gave a very similar project to a bunch of students and most of them spent about an hour completely cutting out the actor's entire body for the entire length of the shot, one tried to fill in the background, and only 2 of the 24 students masked only the actors upper body and head for the 30 frames where his upper body and head needed to pass in front of the street lamp we added to the scene. Two were done in about 15 minutes. The rest were just getting started.