Olywynn mentioned he has a modifier higher than the requirement for the slot. In the case of the other classes that work similarly, you would get a spell slot for that level when you unlock the spells.
Maybe it's a nerf, which I will be fine with, I guess, but I don't think it'd be a good one.
How to answer this without rambling...
Okay, he would have to be an 8th lvl assassin to unlock the base spell slot, even though he gains the ability to have a spell slot at lvl 7. 17 int isn't even enough to net him a bonus slot for 4th level at all ever, all he gets from that is the allowance to cast that circle once he's gained enough levels to do so, so unless he's lvl 8 assassin he should not have any slots yet, though the spells for that level he would have been allowed to pick on lvling up to 7. The same thing can happen to bards or other casters who don't use prepared spells. It is possible to have the base ability score necessary to cast a spell level, as well as the level necessary, while still not having the ability to have any slots for it.
It would help to know what lvl exactly of assassin he is. If he's lvl 7 with 17 int he would not have any slots available yet without an item that grants him one. Comparing this to a paladin or ranger is tricky because they cast prepared spells that come from a complete spell list. It's apples to oranges.
Hopefully that cleared things up, but I feel like I wasn't very clear about it
edit: if he were lvl 7 and had 18 int, he wouldn't have a base slot to cast those spells, but he -could- cast them anyway, as 18 int is enough to grant a bonus 4th lvl slot