OK - first of all, at a year and half old - it sounds to me like he may be more interested right now in girls than in feeding, even if there are no girls around.

Your profile doesn't say where you are from, but because you said he was about 1 kilogram, I'm presuming you are in Europe?

Do you have any way that you could get a live mouse or rat to offer him and see if he'll take live?

If he's not losing significant amounts of weight, I wouldn't be too concerned. I know how stressful it is to you as a keeper, I had a male go on a winter fast that lasted 8 months. Drove me NUTS!