shouldn't my almost 5 month old be holding his head up by now?

My son is 5 months on 29th june. He can hold his head up for a certain amount of time but then he goes all floppy again, he still needs to be supported with his head. I can sit him up leaning against something for a certain amount of time before he starts to fall. I've seen other babies around the same age and I'm starting to think my sons a little slow. What can I do to encourage him to learn ? I was thinking of buying something that he can sit in to encourage him to hold his own head up? He's on solids so maybe a high chair?  hat do you think?