Have you considered that a full school day for a 5yo can be tough and demanding and that he might need a bit of time to adjust?
From what you write, he’s really struggling and tired, which means that forcing (yes, even gently and with your support) him to read will likely have the opposite of the desired effect. You don’t want him to learn that reading is a stressful task, reading should be both a tool to acquire information and an enjoyable activity. If he’s been reading before, I wouldn’t worry, he’ll pick up once he’s settled in at school again.
If you (and the teacher) think a certain amount of reading is important, find other time slots when your child isn’t tired - the upcoming autumn and winter lends itself to cozy afternoons reading on the sofa and once you look at reading as a past time (as opposed to a school chore), you will likely find more suitable times in your family schedule.
If necessary, talk to the teacher and have the “reading assignment” suspended for a bit - or at least not “checked”.
That said, the usual recommendations how to encourage reading are still valid: Make it a positive experience, model reading as a parent, find interesting reading material. And think outside the box, practicing reading skills can mean so much more than books. Board game instructions, the directions on a pack of pudding, (kids) magazines, even the back of a box of cornflakes, the comic strips in the Sunday paper, just to name a few ideas. Have them read the recipe to you while you’re cooking. Wikipedia has articles in “Simple English” (easier to read, fewer “long words”). From personal experience, my kids were always extra determined to struggle through a text if they wanted the information in it.