You don't want to believe it's your DH, but it obviously is, to the point where he's even accused you now and it's as clear as day he wiped the history, too many coincidences.
You didn't want to believe your son, so you asked him multiple times, which is why he changed his story, he would of known you didn't like the first answer, otherwise you wouldn't have asked repeatedly.
So take it as what happened happened, then think about what you want to do to do about it. At the very least check that parental controls are on it before your son uses it again.
There will possibly be other telltale sexual preference and behaviour signs if your DH is gay, but if he is bisexual or bi-curious, possibly not.
Was he brought up in an environment where being gay or bi would of been seen as unacceptable? You know him best, so perhaps you will be able to guage the level of pressure he may have experienced that could of lead to him conforming to convention, rather than being up front about his sexuality. What are his friendship groups like?
Now you know he's at least bi-curious, though he won't admit it, then it's up to you how you feel about it. Do you trust that he's faithful to you? Can you accept it if it stays as fantasy, or would it be a deal-breaker? You know he would rather concoct stories and cover his tracks than be honest, would he cover infidelity like this also?
Unfortunately, and inevitably, your mind is going to be working overtime on this, anyone's would. I'd be closely watching him like a hawk, and seriously worried if he works away or spends nights away from home at times. It's going to be hard, time will tell if it starts impacting on your own emotional health and happiness, you will either get to a point where it becomes a deal-breaker or you may feel that sweeping it under the carpet is doable. Either way, ensure your son is kept safe and not exposed to it ever again. Put your own parental control on all gadgets your son uses, with a password that only you know - if you don't know how, it's time to find out ( if you have a tech- savvy friend, this could be handy, no need to explain details) and learn this as this is the most fundamentally important thing you can do to protect your DS.
Then, work out what it means for you.