Hello Gibson00,
It could be. Low tostesterone is known to be a cause of low energy levels in men. I have been on Lupron for two years now and must admit that I have not had this side effect. This may be due to my regular exercise routine. It would help if your dad did some modest weight bearing exercise and some walking daily. He should also speak to his doctor about taking some diet supplements. I take B12 and that seems to really help. I have seen some articles that say no B12 with PCa but I take it anyway. You may want to do your own research.
Good luck to you and your dad.
Don