Depening on the context of the sentence, but in most cases, He was seen (past participle of see) entering the building-ing of enter is the present participle. He was seen to enter the building does not mean he was with anyone. To always means direction, action or feeling. It can be infinitive but not always.

My native speaker reaction is that only "entering" is correct and/or natural. It is a progressive movement. You could use "to enter" in this case: "He was seen to enter the building every night at 6pm" (a frequency situation), but still "entering" feels better. This example works well with the infinitive: "He was known to like football." Here, we have a general statement of fact and a stative verb (like). The infinitive expresses better this more general, permanent reality. The -ing form (gerund/participle) better expresses movement and activity.

He was seen to enter the building. = Somebody accompanied him to the building. He was seen entering the building. = Somebody saw the process of him going into the building.

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I see. Thanks.