At 8weeks old, babies should still really be getting a good amount of formula- and the formula provides the water that they need.
If he was force weaned early, but is eating on his own, you’ll need to provide moistened foods for him. You can add hot water in a 1:1 ratio to pellets, so a teaspoon of water added to a teaspoon of pellets and see if they absorb enough water to make a soft gruel. Hopefully your brand of pellets isn’t a very dense or hard brand- if it is, I would switch if you can. If you cannot switch brands, you’ll need to experiment and add hot water until you get a gruel/oatmeal/porridge consistency.
Some brands will need quite a lot of water to get to this stage! But if you don’t soften them, you’re going yo continue to struggle with constipation.
I don’t like giving so much fruit to youngsters at this age. Sour crop and other nasties happen. I would reduce the amount of fruit, and see if you can’t get him on something else moist. Chopped hard boiled or scrambled eggs is okay at his age- if you add water to them. You want them kinda soupy. Boiled mashed peas are good with babies this age, as well as mashed sweet potatoes on the thinner consistency side.
Keep checking his bottom for pasty butt- you want to be sure his bottom is free from any stuck on fecal matter, and also ensure it’s not red/inflamed/irritated from straining to poo.
You could always try a few spoonfuls of hand feeding formula, mixed per directions on the pack, and see if he wants a serving at noon and night. But if he was force weaned, the softened pellets may be your best bet.
Keep offering clean fresh water in his cage, and in time he will find it. He’s just little right now, a baby, and needs to be treated like one.