Learn German: Ein dummer Umweg hält ihn wohl auf. - A stupid detour is probably holding him up.

The word wohl is a modal particle here. It expresses a strong assumption or high probability, translating roughly to "probably" or "I assume". It makes the sentence sound more natural and conversational than if you used a standard adverb like wahrscheinlich.

I previously learned to use the Futur I with wohl for assumptions. Why is this in the present tense?

While you can use the Futur I for present assumptions (Ein dummer Umweg wird ihn wohl aufhalten), Germans frequently just use the present tense with wohl when guessing about something happening right now. It is simpler and equally effective.

Why does the adjective have an -er ending in ein dummer Umweg?

Umweg (detour) is a masculine noun acting as the subject (nominative case). Because the indefinite article ein lacks a specific gender marker in the masculine nominative, the adjective must take the strong ending -er to carry that grammatical information.

What verb is being used here, and why is auf at the very end?

The verb is aufhalten, meaning to delay or hold someone up. It is a separable verb, so in a standard main clause, the conjugated stem (hält) takes the second position, and the prefix (auf) gets pushed to the very end of the sentence.