A boyfriend that still has dating apps on his phone is worrisome. He clearly knew they were still there, given that you saw them when he was showing you something on his phone. Even though he deleted them, there is still cause for worry.
Although there may be some truth in him telling you that he was using those dating apps prior to becoming your boyfriend, it’s still a good idea to be diligent in observing his behavior moving forward.
Keep an eye on his phone over the coming weeks. If he reinstalls those dating apps, that is confirmation that he is using them to cheat on you.