Why did he figure it couldn't be detected?
Because the neutrinos interact so weakly that it was for some time believed that it will not be possible to detect. E.g. in the 1934 paper by Bethe and Pierls, where the crossection for neutrino interacting with matter was computed, they concluded that "It is therefore absolutely impossible to observe processes of this kind with the neutrinos created in nuclear transformations."
Was this because he thought it was massless?
No.
I thought the particle was hypothesised in order to maintain the conservation of momentum in a beta-decay. If it was massless, this would have no effect, right?
No, the massless particle carries energy and momentum just as good as the massive one.
To correct you a bit, the neutrinos were hypothesised to explain the violation of the conservation of energy. The observation of the "violation" of the conservation of momentum due to the neutrinos carrying momentum was actually the first evidence that this idea of new "invisible" particle is not that stupid.