Looking for advice

One of my boys has recently lost a noticeable amount of weight. He was on the chubby side when I got him about a month or two ago, at their first vet visit my vet actually recommended that they both needed to lose a little bit. I didn’t change anything in their diet after the visit but I still seen he has lost weight, he is very active and they are out for long periods at a time. He hasn’t had a change in energy levels and I haven’t really noticed a change in appetite either. I’m just worried, does anyone have any suggestions?