Whether its his old problem or something new it looks painful. Poor guy. Hopefully whatever it is they can treat it and relieve his pain so he can walk properly. At nearly 41 he's not spring chicken but he could live another 20 years too.
At least with it being a long term tumor they are very experienced with it. They know what it is and how best to treat him now that its bothering him. Hopefully they can ease the pain and increase his mobility again. It sucks for us humans to walk on an injured leg, I can imagine it being so much worse for a hippo.