Do you mean: "Er hat ... Platz gegeben"?
The answer can be both "einen" and "einem" depending on the context.
However, even though "einen" is grammatically correct, it assumes simply too much in the previous part in order to make any sense in the first place. The omission of the dative object is really hard to justify.
"einem" on the other hand doesn't need that much of prequisits. You just have to have mentioned a group of people previously, and it then means "He made space for one person of the group".