Hmm, it could be worse, my DS used to come back reeking of chip fat and cigarette smoke, it was horrendous.
I agree mentioning that he seems sensitive to perfumes, if he is. But don't make it into a big deal. If he's not sensitive to it, then I would try not to rush to take all of his clothes and wash them too - it's part of his link to his father and you have to detach and understand that is important to him. This is so hard, but you really have to let go and let them parent the way they want to even if you don't agree with what they're doing. Unless he's abusing your son then you have to let them work out their own relationship.
I hope you don't get too much of a bad rap on this thread, it often goes this way, harsh "well, whatever, you're being a controlling bitch" type responses. While I do agree that the dad needs to be allowed to do things his own way, blimey, some empathy for the fact this is massively hard to deal with wouldn't go amiss.
I think that if people haven't been in the situation, if they are still with their child's father, or have a good relationship with an ex who is reasonable, it's hard to imagine what it's like to have to hand over your precious baby for X amount of hours every few days and literally have no say or control over it, even if you know that they are doing things which you don't agree with. There is no other situation like it, if you put your 7 month old in nursery or left them with a grandparent or other babysitter, then you'd be within your rights to explain some wishes and have them be followed. With a reasonable ex or in a happy relationship, you're making decisions together and are usually in agreement. I think some posters as well have a sort of "serves you right" mentality which is just mean and nasty IMO (in my opinion). Lucky them for having such excellent judgement.
So three things - detach from what he's doing. Try not to ask or look for information about it. Don't read things into signs which aren't there - as he gets older you'll probably see behaviour issues emerging after contact visits, it's a combination of the sudden shift between dad's house and mum's house, especially if the environment is very different, excitement to be back, sadness at leaving dad, tiredness, sometimes a sugar crash too. Similar when they seem reluctant to go, it can be a combination, transitions are hard, sad to leave you, happy and excited to go, nervous about new things.
Talk to your DS to help him with the transition. Even a baby can be helped by talking to him. Have a soothing routine when he arrives home which could include a bath if you want an excuse to change his clothes. Babies and toddlers like to know what to expect.
You need to be at least neutral, at best positive about his dad to DS. If he does something which upsets him, never say "Daddy is xxxxx" (mean, bad, naughty, etc) but talk about the behaviour and how it makes (DS, etc) feel.
Don't micromanage or try to control the relationship or how they spend their time. And don't chase him. You make DS available for contact and discuss possible days and times etc. If your ex is messing around with contact times, then get more rigid. Stick to those times (assuming no constraints like odd work shifts) and don't make concessions for him. If he's reasonable, of course, then by all means be reasonable back. You can give him information about DS (he's sick if he eats X food, he gets hyper if he has Y food, he won't eat dinner unless he has a reasonable nap, it's safer for him to be in Z car seat, etc) but then you must leave it up to him to choose to use or ignore that information, however maddening it is for you. Obviously legal things like using a car seat/life threatening allergies etc are different but basically you have to let him work things out in his own way.
Good luck :)