He is probably missing his Dad ,was he close to him, whilst his Dad is away you should pay him extra attention to make up for him not being with him, make sure he gats plenty of outside activities and games, dont focus on time outs and punishments find positive ways to help him . Explain to him about his Dad and hopefully he isnt going to be away for too long explain that to him. Can you ask his Dad to send him letters etc so he knows his Dad thinks about him.Good Luck