My cat is not handling heat as well as he used to

Hi everyone!

My cat is a male 4 year old. He is some sort of Grey Tabby.

Anyway, summer is coming, temperatures are roughly 36 outside and up to 27 inside and he keeps looking for cold surfaces and just lies on them all stretched out.  If he is on the floor or on a table and I just touch him he immediately lies down and stretches. But not the kind when he wants to play or cuddle.

He is still eating normally, and preforming all his bodily functions, and he seems to be breathing normally and seems healthy. But he used to love when its hot, and now he doesnt seem to tolerate it as well.

I am mostly worried because his brother died when he was 1.5 due to heart problems  so I was worried if it runs in the family.

I am currently too broke to take him to a vet and I dont want to cause any unnecesarry stress on him either.

Oh and one last thing of note, he gained some weight since last year so maybe that has something to do with it as well?

It didnt happen rapidly, it was gradually over the course of the entire year. He is now 5kg and he used to be roughly 4.2

But he seems natural at 5kg, he used to be quite skinny at 4.2.

Any answers or experiences are appreciated, thank you!

Most cats seem to be feeling the heat this year. I just spent the morning at the local animal shelter, and just about all of the cats were all stretched out and dozing by 10 a.m..

Heart problems and excess weight could certainly play a role, but so could a prolonged hot spell and/or high humidity. Is he still quite active at night and in the early morning when it's cooler?

Mogli can't take the heat (he's just turned 3). We open everything up at night, then close all the windows and doors before the heat of the day to trap the cooler air inside, plus close the shutters on the south side of the house. He enjoys lying in front of an oscillating fan, too. A stationary one would put him in too much of a draft. Some people put ice cubes in the water bowls to encourage drinking.

He is a bit more active when its colder, yeah. And if we get him to play he does it  the same intensity, except he lies down faster than he used to.

It started exactly when it got hotter so I`m sure its because of that, I was just worrying if the heat could cause problems with his heart if by any chance he had a heart condition?

Also I wouldnt necesarily say he has excess weight, he just has more weight than he used to. I did the check of the usual "signs" such as feeling his ribs and spine, checking if his hips still have a "dent" etc, and he isnt really overweight, he just used to be really skinny.

He only has some flappy skin near his back legs, but he used to have that even when he was at 4kgs so I dont think thats a problem.

Here is a picture of him just in case.
And thank you for the advice!