I don't think he was a Muslim convert; that seems to have been the result of an Israeli newspaper that got suckered by the Sam Hyde did it meme that's gone around after several of these mass murder/terror attacks (see the Bustle link in the references above)
Now ISIS is claiming that he was a convert from months ago
- ISIS has lied in claiming atrocities before
- More background on Marilou could shed light on this
- Overall, at this time, I find it unlikely