URGENT Gecko Heat Mat Question?

I have a blue heat lamp for my leopard gecko that stays around 85-90 (not sure about the cool side temp at the mo as not got enough money to buy another digital temp gauge but will have in a day or 2) I am worried because he WILL NOT come out of his hide unless I turn the light off and turn the heat mat on so I do this in 12 hour cycles to give him the day/night scenario I am told he needs (and have stayed up all night - insomnia), however, since I put the heat mat UNDER his viv as instructed it does not reach these temperatures at night - he still spends time in his cooler side hide so he doesn't seem to mind but I came home last night to find it was 66F on the hot side and I know that this is not ideal however do not know if this is harmful in the long run? Should I put the mat back inside the viv against the manufacturers instructions? Should I plug it straight into the mains to heat it higher (at the moment it is connected to a thermostat that maxes at 93F but obviously through the wood of his viv it is not conducting to that temp) the mat is the right size for the viv. I have looked and looked and seem to be the only person experiencing this problem so know I am doing something wrong. My Gecko is at least a year old as he is around 8 inches long, the reptile shop I bought him from could not tell me exactly how old... I have had him nearly 2 months and have no idea how thick his tail is as am too scared to touch it to find out!!! I handle him and he is ok in himself, his tail is not as thick as it should be because he took a while to start eating properly but has finally got a heathly appetite and his tail is getting bigger, although he is the worst hunter in the world, i have to completely clear his viv of all objects (30 gallon) for him to eat otherwise he just ignores his crickets, however, in an empty viv he will eat immediately. Please help, now he is eating properly, i think once I get the heat right he will be happy and healthy and that's ALL i care about!!!