קומי שאי את הנער, "arise and lift up the lad, etc.⁠" The angel meant that after Ishmael had drunk from the well Hagar should lift him and lead him by her hand.

According to the sequence of the story as related in the Torah Ishmael must have been between 16 and 17 years of age at that time. He was born when Avraham was 86 years old (16,16). Yitzchak was born when Ishmael was 14 years old as the Torah describes Avraham as being 100 years of age when he was born. Two more years (minimum) had elapsed until Yitzchak was weaned. This means that this episode took place when Ishmael was at least 16 years old. This raises the question why Avraham placed a 16 year old on his mother's shoulder when he expelled them? (verse 14).
Should you argue that the words ואת הילד in verse 14 do not refer to the words על שכמה, "on her shoulder,⁠" the fact that the Torah describes Hagar as "throwing the child under one of the bushes" (verse 16) makes it clear that she must have been carrying him first. Alternatively, she had held him in her lap and when he cried too much she threw him off her lap. Another difficulty in this verse is that the Torah suddenly refers to Ishmael as הילד, "the child,⁠" although previously (verse 12, God speaking) he had been referred to as הנער, "the lad, the adolescent.⁠" The word ילד is usually reserved for very young children, certainly not for a sixteen-year old boy. (compare ויגדל הילד ויגמל, "the child grew up and was weaned.⁠" This suggests that already when being weaned the description ילד no longer applies, verse 8) Furthermore, the angel had told Hagar to lift up the נער, "the lad,⁠" not the ילד. In addition we see that in verse 20 the Torah speaks of "God being with the נער, "the lad,⁠" and that "he grew up.⁠" We can only fall back on the principle of אין מוקדם ומאוחר בתורה that the Torah is not obligated to relate events in their chronological order.