"Is he dancing?"
The pronoun is only necessary with third person singular with this meaning.
But you could use "Tanssiiko?" like this:
"Hän tanssii kanssani." (He is dancing with me.)
Unfortunately, the app doesn't have tips tied to the lessons, though the website does. The conjugation endings are taught in the Family tips. All the tips are available on Duome here, and I have links to them on Duolingo here.
The infinitive form of this verb is tanssia, and it means 'to dance'. We delete the last a to get the stem tanssi-, and then we add endings onto this stem.
When the subject is minä, the verb is tanssin.
When the subject is me, the verb is tanssimme.
When the subject is sinä, the verb is tanssit.
When the subject is te, the verb is tanssitte.
When the subject is hän, or any other 3rd person singular noun, you take the last vowel of the stem and double it. So tanssii.
When the subject is he, or any other 3rd person nominative plural noun, the verb almost always needs to be tanssivat.
When you want to ask a yes or no question, take these verb forms and add the suffix -ko.