Avraham built an altar;” Yitzchok did not participate in that activity; the reason that he did not do so was not that he would not have been willing to do so, but his father had hidden him so that Satan would not be able to tempt him, as he had tried to tempt Avraham not to carry out G-d’s request by hurting him so that he would be unfit to serve as an offering.
"ויבאו אל המקום. ספר הכתוב שאחר שידע יצחק שיש סברה שהוא יהיה השה וראה שבאו אל המקום ושם בנה מזבח וערך עצים הוא לבדו ולא שתף את יצחק לעזרו במעשים אלה, מזה ידע בברור שהוא נבחר לקרבן שע"כ לא יעבוד בו שום עבודה, ובזה אם לא היה מסכים אל המעשה, לא היה מניח שיעקדנו אביו אחר שידע בברור שעקדו ע"מ לשחטו, והוא היה בן ל"ז ואברהם איש זקן, בכ"ז מסר א"ע ברצונו לקרבן, ובזה נשלם הנסיון גם מצד יצחק..."
"And they arrived at the place. The written text tells us that after Yitzchak surmised that he will be the sheep and saw that they came to the place, and there he [Avraham] built an altar and prepared the wood by himself and didn't share the work with Yitzchak, to allow him to help in these acts, from here he [Yitzchak] knew with certainty that he was chosen to be the sacrifice and for this he [Avraham] didn't work him in any labor [I believe this is related to the din of "on which no yoke has been laid"] and in this if he didn't agree to this act, he would not have permitted his father to lay him on the altar after knowing with certainty that he was laying him in order to slaughter him, and he was 37 years old and Avraham was an old man, even so he gave himself willingly as a sacrifice, and in here was completed the test from the side of Yitzchak..."