Responses
He does remember him, but like he said in the previous chapters before the flashback (and also in some earlier chapters), he thought the boy had died because of the lack of support/funds since he got stuck in the rift.
That's why before these flashbacks when he was talking to Seo Mingi, he wanted to go to the Prometheus laboratory to find evidence about the boy

Now does he forgot about that boy? Like mc said after being inside the rift the memories are disoriented, but he didn't mention about the memories before entering the rift. And if he did remember the boy, why doesn't he search for him? Ik the situation of mc, but even so they would try right?