Why would you think of putting a three and half month old in a walker when he can barely hold his neck and can't even sit??
Ideally speaking a walker is not recommended at any age whatsoever. It endangers children by way of making them mobile at an age when they do not have much body control and do not understand the risks of bumping into objects or falling down.
Next Steps
If you are still interested in putting your son in the walker you have to wait until he starts standing with support and that too under strict supervision