Does anyone know about taking care of a frog?

My son's little friend gave him a pet frog, because they had like 4 of them and didn't want all of them. We have him in a aquarium, with mulch on the bottom, and a dish for water since I think he is semi aquatic. He has a little half log type thing to go under, which is where he is most of the time. We have been buying crickets at the pet store for him to eat. The past couple of days, he has seemed sick I think. I even tried to move him around a little, and he barely moved his legs but didn't jump or anything like he usually would if someone touched him. My husband said he thinks maybe there isn't enough warmth for the frog (I should mention we have the aqaurium on an end table about 4 feet from the front door). I am not experienced with taking care of animals, as I am not fond of pets in general. To be honest, I can't even keep plants alive. Anyone know any more about this or have any tips on caring for a frog? I beleive he is a leopard frog, btw.