May sound silly but try watching some episodes of supernanny - I follow her on fb and when my daughter goes through behavioural struggles I find it so helpful. These often come about when there is a change in her routine. It’s totally normal, and for how young your son is he needs structure and boundaries to feel safe.. it’s really tough at first but I promise you it’s worth it! Supernanny episodes really motivate me as it shows it’s normal and the improvements DO happen. I can also confirm with my daughter that is nearly 6 as a single mum, we have been through many rough spells with her behaviour but follow through, persevere, be kind and fair. I have also found “gentle parenting” and child behaviouralist accounts on Instagram very helpful - I will tag some for you!
You should try spending more quality time together. Especially now that you’re working. He sees less of you. You can go on your phone in emergencies or when your alone, when he’s sleeping.