I played Lost Spearman from my hand, my opponent kneeled a card that has 2 Influence printed. Lost Spearman returned to my hand, then I paid another gold to play him again. My opponent who knelt the card with 2 printed Influence but only used 1 of it, does he have a "floating" influence that he can use to bounce my Lost Spearman back into my hand? Or once he uses the 2 Influence, thats it, it's done.

You can't "accumulate" influence. If you pay 3 influence (for example using The Red Keep) for an effect that requires only 1 influence, the other 2 are lost.