Thank you for your question
My name is ***** ***** I have been a solicitor for more than 30 years
In fact the law says that children should have a relationship with both parents and the courts will enforce this if necessary
Despite what she may think his ex cannot simply dictate when he sees the children.
This is something that they need to discuss and agree together- possibly using Family Mediation
But if no agreement can be reached then your son can apply to the Court for a Child Arrangement Order setting out when the children will be in his care.
If mum has always been the main carer then this will continue - but your son should be aiming for alternate weekends (friday to monday if possible) plus one evening /night in the week plus up to half of the school holidays
he can find out more here
If the children are over 11 their wishes are of great importance in terms of what the arrangements should be.
So far as the house is concerned his ex is NOT allowed to change the locks - and your son is entitled to return there whenever he wishes breaking in if necessary (although it may be unwise to do this)
He only has to stay away if there is a Court Order in place excluding him from the property
He is free to return to the house when his ex is not there and remove the tools that he needs if she is refusing to let him collect them - although it would be wise to give a deadline first
I hope that this is of assistance - please ask if you need further details