Here is the correct declension:
Make sure you're looking at Polish not Azeri one.
'Pomidor' is one of the few words which can be used both in dative and accusative form here, so 'chłopiec je pomidor' and 'chłopiec je pomidora' are both correct.
And yet, it is correct. See e.g.
This article even claims that a couple of decades ago, 'je pomidor' was the only acceptable form.
I'm not saying it's incorrect. It just sounds awkward and it's definitely a more obscure of the two forms in the modern day spoken language.
It's a single tomato, it needs an article.
Googling "I am eating tomato" results almost entirely in pages about eating tomato soup.
Is there a specific way of turning a singular word into a plural? Or is it different for every word?
By this, i mean, how English changes tomato into tomatoes by adding "es" (this isn't the best example because it adds an e as well as an s, but you hopefully get my meaning)
It's not that simple, but also not that difficult, there are rules ;) Take a look here: http://mowicpopolsku.com/polish-grammar/cases/nominative/#noun-plural
Also "Jem arbuza" (I eat a watermelon) and many other tasty things of Masculine grammatical gender. It may sound strange to speakers of some other Slavic languages, because only animate nouns of masculine grammatical gender change their form in Accusative there, while inanimate nouns have the same declension as Nominative.
Thanks for the link, it's very useful, there are answers for some hard moments that may happen while studying Polish. Yesterday I was googling jem+kind of food, but unfortunately most of them were Feminine (cebula, cytryna erc) and then I stumbled upon "jem arbuza" and one page where the teachers of Polish discuss this topic. https://e-polish.eu/forum-metodyczne-dla-nauczycieli-jezyka-polskiego-jako-obcego/jem_arbuz and some other pages, including one that was mentioned above here (ekorekta24.pl...).