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I don’t feel like the best person to give advice, but it sounds to me like you probably have enough foreshadowing. You have Jake somewhere where experiments on done on convicts, while Felix is a convict who had experiments done on him. You have the little bits of hinting, etc. Also, until you get close to the climax, it’s pretty expected for new characters to appear. Since you say once they escape the prison, they go on an adventure together, it sounds like they meet in the first half or so at least. I’d say you have enough set up already to just introduce him however you want. If beta readers further down the line have issues, then you can look it over again.