I Need Help/Does anyone have expierence with MPAL?

Hello dear members, dear parents, I feel totally helpless. I hope to find help here. My 4 year old son Mica was diagnosed with leukemia (AML) in December 2021. He was then given chemotherapy until June 2022, remission was achieved and we were discharged home. In November 2022, we had to pause the long-term therapy with the medicine Tioguanine because of a corona infection. After 2 weeks at the blood sampling they found that the leukocytes were much too high. Then at bone marrow aspiration the diagnosis: the leukemia was back. This time a TALL was diagnosed. Thereupon he received chemotherapy again. In February 2023 he received preparatory chemo, total body irradiation and then in March he had a stem cell transplant. He had almost no side effects and handled it well. He was told that he had 100 percent donor cells. The only thing was that he was infected with the Epstein-Barr virus from the donor and was then given a medicine against it (Rituximab) 4 times in a few weeks. On day 174 after the transplant, he was told that his B-cells were too low and that this was due to the rituximab. He was then given immunotherapy (Protein) to increase the B cells. He reacted with fever during the infusion. After 2 hours the temperature was normal again. We were allowed to go home in the evening, then he got fever again the next day. The temperature fluctuated for 5 days between 37.5 and 38 degree. He was then given antibiotics but the temperature did not go down. When they did the blood sampling, they found out that 1-5 percent of his own cells were back. On day 186, another puncture was done. It was found that the leukemia came back. That was about 3 weeks ago. Now it was said that it is a mixed-phenotype acute leukemia (MPAL) with 2 thirds AML and one third ALL. Since then he gets an outpatient chemo with Venetoclax. Not all the bone marrow results are in yet, at the moment we are still waiting for them. He is doing well so far. Does anyone have experience with this rare childhood leukemia? What therapies, studies or new medication are there for it? We are from Germany and are being treated there. Does anyone know a clinic in other countries that specializes in this? I am really desperate. I would be so grateful if someone can help us