Hi everyone and thank you all for your very interesting anwsers !

Since I added a little light (leds 30W), turned on from 8am to 10pm, it seems more active and he just ate a bit of fish filet I cooked for myself (well I didn't cook his little part). He didn't eat much but enough. I am happy with the terrarium now, I may change it a little in some weeks/months when I will change the "soil" made of pine barks, but he seems better now. He has indeed a place to swim but I am surprised he doesn't go lay in there at all. I think my conditions are now okay, I will try to give him various preys but he was used to eat little fish for 2 years so it's a long road before he accepts frog legs or mice, but I have read that they are way more nutritious. For now, I will only put a bit of vitamines on his fish and it's gonna be fine I guess.

I will try these worms, I have also read that one species is toxic, but it's not Canadian worms. Thank you for your answers, I will try to take a pic of him to show you, but he is kind of shy, very curious but very shy ahah