Legally you both have the same level of parental responsibility for your youngest son, and therefore either one of you can offer him full care. However, your ex has no right to withhold your oldest child from you and you, as the person with PR, are free to go and remove him from the ex's care if you choose to. There is nothing that the ex can do to prevent that, unless there are genuine safeguarding issues. You can also, if you choose, remove the youngest child as you share PR for him and without a court order stating where the child lives, either parent can have that child in their care.
However, that said, I would not suggest that you remove the children from their home to live in a hotel. If you were going to remove them and take them to suitable accommodation, such as a 2 bedroom property, that might be different. You cannot remove the youngest child from England & Wales without the consent of his father or the order of a court.
If you have accommodation in Scotland - for example with your mother who could help to provide you with support - you might wish to apply to the court for the following orders:
- Child arrangements order for the youngest child to live with you
- Specific issues order for both children to be returned to your care
- Specific issues order for you to be given leave to remove the youngest child to live permanently in Scotland
These orders can all be made on form C100 - all at the same time - see here: https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/734888/C100_eng_0818.pdf
As you want the children returned to your care immediately, you would be making the applications as a matter or urgency and could therefore skip mediation - see the form for further information on exemptions.
The fee to file is £215 unless you are on a low income or benefits, in which case you may be entitled to fee remission - see here: https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/634329/ex160-eng.pdf
Does that assist?