Ray of Enfeeblement and Clumsiness, for example, are great spells pretty much all the way up. And, even better with a cheap rod or a little metamagic. A -5 to attacks and damage or a -5 to AC and Reflex saves is no joke.
Yep, those are good. Don't think any of my players have them.
Ray of Weakness/Dizziness/Exhaustion are other good examples. There's also the classic Slow (many targets mean someone is likely to fail a save), Web and Solid Fog -- the latter 2 not having saves at all.
The last session the player in question "owned" the combat against a Rage Drake (to the extent he could). Surprise round, the Beguiler goes first, tags the Rage Drake with Ray of Dizziness. The drake could still charge up to its speed and pounce (against the Barb//Beguiler). The next round... I forget, I think the Barb//Beguiler struck the rage drake with his sword, or maybe I'm getting their turns out of order. The rage drake, limited to a single standard action, still pounced on another character, dropping him to 2 hp's. The Barb//Beguiler then tagged it with Hold Monster, paralyzing it. It made its first subsequent save, but it took two rounds to do so (the re-save is a full round action; being limited to one standard, it took him two rounds to perform). He was dead before he acted again.
My party has been Webbed before, but I don't think any of them have the spell. They are going to get webbed again soon, and it is going to hurt them. (3.0 Hasted Spell Weaver will Web them, buff himself a lot, then with his hasted extra standard action he'll hit them with Cloud Kill. Then, after they've taken some Con damage and exit the could kill, the spell weaver will whip out the disintegrate. It should be a learning experience.)
Honestly, though, your player needs something of a paradigm shift. A low level spell is a weak magic -- it's either a significant effort from a weak mage or a trivial effort from by a powerful one. Against mighty foes, or foes strong against a particular type of effect, they will not do much. That's the conceit of the game.
If he's wanting something where a spellcaster's abilities more closely just track his firepower, he either wants a system more like Mutants and Masterminds or to play something like a Warlock, which has fewer abilities that aren't differentiated by level. Alternatively, something like a Psion with the augment system fits this paradigm better: you have a set of effects you can use, and you decide how much effort (power points) to put into each one. Spontaneously Heightening a spell has similar results.
I'll point him toward Heighten. He is a spontaneous caster, after all. He doesn't really have the feats, though. At 12th he's going to take Arcane Strike, and at 15th, Mindsight (he has a level of Mindbender).
All that being said, the aforementioned paradigm does sort of go out the window with the great gems of low level spells, but what are you gonna do ...
True, true. As a beguiler he doesn't get to just pick whatever he wants, however. But he's still got a pretty awesome spell list (especially since I reintroduced what is basically 3.0 Haste as a 5th level spell, and allowed him to have it on his beguiler spell list for free, because they get Haste).
He's been able to cast 3rd level Haste for over a year now, and he's maybe only ever cast it once...