Presumably you have discussed this calmly with him ? What does he say he is doing on Whatsapp in the early hours of the morning ? And how did you find out about that ? He may try to make a big fuss about you snooping on him, to avoid discussing who he is talking too so late. Presumably you're not suspicious because of the Whatsapp conversations, or you wouldn't have looked and found them - what else made you feel so suspicious ?