You fix them when you do the finishing up. You have to seam the sleeves in someway, and use a new yarn end when you continue on the back sts so take those tails to close up the gaps.
A little better way to knit this sweater is to work on the body sts only first, then go back to the sleeves. Work across the front sts, put the sleeves on holders, CO 4 sts to the R needle (switch it to the L hand if you want to use a knit CO) then work the back sts, put the 2nd sleeve on a holder, CO 4 and finish the rows. Then just work on the body stitches until done. When you go back to the sleeves, pick up sts in the 4 that were CO, plus one before and one after them, then dec the extras on the first row. You can work the sleeves in the round too with ML or dpns to avoid seams altogether. If there's stitch gaps at the underarm, use the tail yarn from the sleeve sts to close them.
sue- knitting heretic