Officially he is a suspect. The prosecutor filed a national warrant for his arrest(they can do that over here), but since then a district court has found that there is reasonable grounds for him to be detained, Assange appealed this verdict twice, first to the Svea Hovrätt(Svea Court of Appeals) which rejected the appeal, he later also appealed the rejection to the Swedish Supreme Court, the Supreme Court as well rejected that appeal.
So yes there was initially some doubt over the validity of the european arrest warrant(because the legal systems work somewhat different, swedish prosecutors have some powers their brittish equivalents doesn't) but that has long since been corrected.
The Swedish government or legal sytem CANNOT guarantee that he won't be extradited prior to receiving a request for extradition because it is largely a judicial matter, the government can overrule the court under some circumstances, such as if there is considered to be a risk that the extradition target risks torture or death if he is extradited.
On the other hand if Assange is handed over to the Swedish authorities both the Swedish authorities and the UK authorities would have to agree to extradite him to the US for that to actually happen because according to the framework for the European arrest warrants you cannot forther extradite somone without the permission of the original extraditing country.