Radak: He did not request an Os of Hashem's ability. That is forbidden, like Rav Sadya Gaon said. Rather, he wanted to test if he is proper for a miracle like this.

Malbim (38): The second test was for an Os that He would save Yisrael miraculously, with few men. Nisayon is to know whether the one tested can rise to a higher level than he is at now, in this case from natural to miraculous.

Malbim (36) #1: He wanted to see an Os against nature. First he thought that the ground should get more dew, due to its cold nature. Afterwards he thought that perhaps the wool should get more dew, because it is soft and absorbent.

Malbim (36,38) #2: The two tests were parables for the salvation. First he was unsure if his small merit could save Yisrael, who were idolaters; he said "Im Yeshcha Moshi'a b'Yadi" (36). He asked that dew, a metaphor for influence from Shamayim ("Ehyeh cha'Tal l'Yisrael" - Hoshe'a 14:6) coming on the wool, even though the ground would be dry, i.e. Yisrael were not worthy of this Tal. Refer to 6:39:1:2. ¹