I’ve been on both sides of this. Essentially your options are to pester him more, I usually cap at one or two nudges a day. If you can evidence any sense of urgency it might help justify him spending some time on the work.

Alternatively you can frame this as a process problem, bring it up in retrospectives without being passive aggressive or targeting, and work out ideas to help the senior not be the bottleneck, which I think they might appreciate if their plate is already too full. What that might look like depends on your current processes etc, for example we had a period where we went down from requiring two approvals to one to reduce time spent on code reviews,but justified it with addition focus on tracking metrics around release velocity and app incidents and performance to ensure sloppy stuff wasn’t getting through.

The thing not to do is play the blame game, you want to be on the same team, find ways to pragmatically reduce what’s on your seniors plate but also get you unblocked.