Why does he answer my whatsApp only when he wants to?

Hi there. Answering someone whenever one wants to can be looked from different perspectives. It maybe because he needs time to think what to answer or maybe he gets caught up in certain situations just before he can reply back. There are also distractions and at times, people might just not read them completely or think that they have replied but actually have not. It all, also, depends on the frequency with which he does this. I think it will be good to ask him whenever you feel is the right time as I do believe that this is bothering at times.