He's 8, in the absence of abuse it would be unlikely that he would be better off without that connection and time with his Dad. If I stuck to what my Autistic DC want we'd never leave the house and yard. Better to find ways to support him with whatever he's struggling with here, transitions, routine changes, feeling overwhelmed by being outside his comfort zone, sensory issues could all be at play. Giving him the support he needs to feel less overwhelmed may help make this easier on everyone involved. We're not separated but the same general principles apply to our Autistic DD and DS to get through life without it overwhelming them.
It sounds like he might struggle with transitions and changes to routine. These are often very hard for Autistic children. Is anything being done to support him to feel less overwhelmed by the change and transition calmly?
There's no one fits all of what will help, but there are various strategies that can be tried. Things like a chart with where he'll be going for each day of the next week. Gone through at start of week, then a discussion each night of what will happen the next day, every night at both of his homes. If it's time at Dad's .... time his Dad will pick him up, when will he be coming back to mum's, maybe discussing what he wants to take, how much fun he'll have with Dad. Forewarning of change can help.
A social story might be helpful, his Dad can easily make one using Google images or photos of DSS, though my DS7 really didn't like it when I did the later.
Using school as the transition point where possible could ease the feelings on going from 'home' to his Dad's place. If he has specific possessions he needs to feel safe, bringing those might help. I mean safe in regards giving him a feeling of belonging to both places. Feeling Dad's home is another safe place where he's comfortable.
Is there anything particular to his mum's place that replicating might help? For my Autistic DC anything that can be made similar would help a bit. One especially needs things to be kept as unchanged as possible. Always used the same plates, same cups, same coloured bed linen, same towels, same crockery, same toothbrush and toothpaste, needs his room to always be the same. I'm sure you get the idea. He really struggles with change, even if it's something he really really wants to do the sensory pressures of being somewhere different, doing something different, being out in the too loud, too bright world, the feelings changes give him can all overwhelm him.
Is his routine the same in both houses? DS 7 gets very upset by changes in his routine. He copes better if his morning and evening routines are kept to as much as possible and specific things like down time and screens occur at the same time each weekend and holiday day. We can't always accommodate that which is where the discussion at each dinner and breakfast of what's happening that day can help him cope better with changes.
What would I suspect ease going from one house to another for our DC would be to get tablets in the car or once they get to Dad's place. For a specific amount of time. This is a safe place to escape the overwhelming world and recenter for a lot of Autistic children from my Autistic parents group. Though some children find this really helpful to regulate for others it does the exact opposite. Overtime and with the right supports in place this should become easier for all involved.