There's a lot to unpick here.
Firstly, tell mutual 'friends' you don't want to be informed of your ExH and his DP life. Let them know how unhelpful this is for your wellbeing. If you're looking at their social media, which it sounds like you are, then give your head a wobble and stop. Their relationship is nothing to do with you. It is only adding to your unhappiness and negative feelings towards them, which will end up impacting on your DS.
You say your DS gets asked sometimes to go on holiday, other times he doesn't. This is perfectly normal in a blended family. Especially if ExH DP is financing this, or you're not allowing DS to be taken out of school during term time. You and ExH need to set an expectation for DS that he won't be included in everything, that's just a fact of life. You could ask ExH when he will be making up that contact. Sadly, if he doesn't, you can't force him.
Reducing contact to so little isn't in DS best interest, especially given his age. You could speak with ExH and frame it as DS wanting more 1:1 time with dad, as well as additional time outside the 1 day a week. Could he go for dinner? Dad take him to an activity after school? Stay over a full weekend rather than just 1 night?
The activities for toddlers is difficult to get around. Perhaps suggest to ExH that DS would like to do a certain activity with dad on their next stay? Don't mention this to DS, to avoid disappointment. The activities may be booked and paid for by ExH DP. It doesn't make it OK that she isn't considering DS, but it's up to ExH to prioritise and provide for his son, not the new DP.
You can't dictate the set up in their house based on what you deem is fair. The 2 toddlers are living there 100% of the time. DS is there once a week. You haven't had a conversation with ExH about this, and we all know a 7 year old can get the wrong end of the stick. In my eyes, you're taking what DS said as gospel and have got upset over a potential non issue. Maybe the 2 toddlers will share the night DS is there? This would still give DS their own room when they are present.
ExH DP likely buys her kids things the other 6 days a week. She probably cuddles and kisses them and tells them how much she loves them the other 6 days a week. Should she start treating them differently 1 day a week because DS is present? If she is only doing these things when DS is present, then it's cruel. If this is normal behaviour for her, why should she become a different person because DS is there? She can be more tactful about it, but you're only hearing things from a 7 year olds perspective.
Your post, while heartbreaking that your DS feels this way (although I am also sensing your emotions around this are effecting him too), comes across as bitter and jealous if I'm being blunt. Who cares why ExH is treating DP differently than he treated you? Who cares how soon after divorce he moved on and had more kids?
The only thing you should care about here is DS. You need to have a conversation with ExH and not frame it as how awful him and his DP are and how unfair you perceive things to be, but instead that DS is upset and you want to understand the actual situation.