IME toddlers have phases of eating loads then cutting right back. He is probably getting enough even though you feel it isn't much.
If he is holding out for fruit at dinner time you might want to do no-fruit or limited-fruit for 3 or 4 days. Fruit is healthy so you obviously don't want to cut it out completely but if you only gave a very very small portion for 3 or 4 days (maybe a week) he would likely start eating more variety again. I did this with mine, only once, I noticed how he'd gone from eating everything to holding out for or demanding fruit. He didn't like the idea of limited fruit (I really did limit to tiny portions) and kicked off big time, crying, knocking on the cupboard door where he knew dried fruit was kept, I offered a hug, told him I understood how frustrated he was, that I knew he wanted fruit and of course that fruit is tasty, but that it was my job as his mummy to make sure he had a balanced diet with a range of good foods. I never shouted at him, or told him off for his tantrum like behaviour about wanting fruit, I just sympathised and cuddled but I held firm on my decision. After 3 days he was back to eating properly again. I kept fruit portions relatively low for a while to make sure we didn't go back to too much fruit but after not long I could increase and he was fine.