stewie is right. My ex-H is similar to this - no interest in him from the point I got pregnant to the point I left (he was 2.5 then). In fact, he wanted me to abort him even though he was planned (long story but ).
Anyway, a few months after I left he started to see him a bit more, and now has him every other weekend. He comes home spoilt, tired and with "stuff". Now we have enough money to buy him what he needs (as opposed to wants, despite ex refusing to pay a penny of maintenance) but ex tries to bribe him, possibly partly out of guilt, mostly because he's an utterly crap father and doesn't know what else to do.
When my LO asks me for stuff that I refuse and he comes up with the, "daddy gets it for me" thing, I say that I understand that, but I'm not Daddy, and I love him SOOOOO much but that doesn't mean I'm able to buy anything he wants. It's a bit of a constant battle but in the end I recognise that he loves me and his stepfather immensely and would prefer to be "home" with us than off with ex, even though he gets excited about going and getting new stuff.
It's so easy to get angry about these things and I totally understand it, but the only way to deal with it is to rise above it (as really hard as that is) and understand that your children will recognise that his more expensive gifts do not mean he loves them more than you. Much as they love stuff, it's what you do with them that counts and it's the fun stuff we do with our little boy that makes him really happy - and that costs nothing!
My deepest sympathies though